LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


THE  HUMAN  MECHANISM 

• 

ITS  PHYSIOLOGY  AND  HYGIENE 

AND  THE  SANITATION  OF 

ITS  SURROUNDINGS 


BY 


THEODORE  HOUGH  AND  WILLIAM  T.  SEDGWICK 

Professor  of  Physiology  in  the  Unl-  Professor  of  Biology  in  the  Massa- 

versity  of  Virginia ;  sometime  In-  chusetts  Institute  of  Technology ; 

struclor  in  1  ersonal  Hygiene,  Author  of  "Principles  of 

Boston  Normal  School  Sanitary  Science  and 

of  Gymnastics  Public  Health"  etc. 


The  fundamental  conception  of  the  liv- 
ing body  as  a  physical  mechanism  .  .  . 
is  the  distinctive  feature  of  modern  as 
contrasted  with  ancient  physiology 
HUXLEY 


GINN  &  COMPANY 

BOSTON  •  NEW  YORK  •  CHICAGO  •  LONDON 


ENTERED  AT  STATIONERS'  HALL 


COPYRIGHT,  1906,  BY 
THEODORE  HOUGH  AND  WILLIAM  T.  SEDGWICK 

ALL  RIGHTS  RESERVED 


GINN   &   COMPANY  •  PRO- 
PRIETORS •  BOSTON  •  U.S.A. 


PREFACE 

The  authors  of  this  work  believe  that  extensive  and 
fundamental  changes  must  be  made  in  the  elementary 
teaching  of  physiology,  hygiene,  and  sanitation,  if  these 
subjects  are  ever  to  occupy  in  the  curriculum  of  education 
the  place  which  their  intrinsic  importance  requires.  This 
text-book  is  a  contribution  toward  effecting  these  changes, 
and  has  been  prepared  both  as  a  demonstration  of  what  the 
authors  believe  to  be  needed  and  as  a  practical  aid  toward 
securing  the  end  in  view. 

The  health  and  efficiency  of  the  human  body  have  rarely, 
if  ever,  been  more  highly  esteemed  than  they  are  to-day, 
and  yet  no  subject  of  similar  importance  is  so  gener- 
ally neglected  in  the  schools,  or,  when  taught,  taught  less 
effectively.  Several  causes  have  contributed  to  this  curi- 
ous state  of  things,  but  undoubtedly  one  of  the  most 
important  is  that  the  teaching  has  been  too  largely  ana- 
tomical and  too  remotely  connected  with  the  activities  and 
problems  of  daily  life. 

In  the  present  text-book,  anatomy  has  been  reduced  to 
its  lowest  terms  and  microscopic  anatomy  or  histology 
touched  upon  only  so  far  as  seemed  absolutely  necessary. 
Space  has  thus  been  gained  for  more  physiology  and, 
especially,  for  more  hygiene  than  is  usual,  and  also  for  the 
elements  of  sanitation,  —  a  new  and  comparatively  easy 
subject,  but  one  of  the  very  first  importance  in  all  whole- 
some modern  living. 

That  point  of  view  which  regards  the  human  body  as  a 
living  mechanism  is  to-day  not  only  the  sure  foundation 


iv  PREFACE 

of  physiology,  hygiene,  and  sanitation,  but  is  also  surpris- 
ingly helpful  in  the  solution  of  many  questions  concerned 
with  intellectual  and  moral  behavior.  This  view,  therefore, 
we  have  not  hesitated  to  expound  and  emphasize.  Avoid- 
ing that  form  of  physiology  which  looks  chiefly  at  the 
organs  and  overlooks  the  organism,  we  have  constantly 
kept  in  view  the  body  as  a  whole,  in  order  that  physiology 
may  become  the  interpreter  of  the  common  physical  phe-  , 
nomena  of  the  daily  life  and  find  in  hygiene  and  sanitation 
its  natural  application  to  conduct. 

We  believe  with  Matthew  Arnold  that  "conduct  is 
three  fourths  of  life,"  and  that  this  is  no  less  true  of  the 
physical  than  of  the  moral  and  the  intellectual  life.  We 
therefore  make  no  apology  for  fixing  upon  conduct  as  the 
keynote  of  this  work,  and  the  right  conduct  of  the  physical 
life  as  the  principal  aim  and  end  of  all  elementary  teach- 
ing of  physiology,  hygiene,  and  sanitation. 

In  those  portions  of  the  book  devoted  to  public  hygiene 
and  sanitation  the  authors  have  kept  in  view  the  importance 
of  this  subject  in  all  education  for  good  citizenship.  Sani- 
tary science  and  the  public  health  can  be  advanced  only  as 
they  are  supported  by  an  intelligent  public  opinion,  which 
appreciates  the  nature  of  the  problems  involved,  the  frequent 
duty  of  subordinating  personal  liberty  to  the  public  good, 
and  the  importance  of  rendering  hearty  support  to  public 
oificials  in  the  discharge  of  difficult  and  often  delicate  tasks. 

It  has  not  seemed  wise  to  include  in  the  text  exten- 
sive directions  for  laboratory  work.  The  opportunities  and 
facilities  for  such  work  vary  to  such  an  extent  in  different 
schools  that  the  largest  discretion  must  here  be  left  to  the 
teacher.  Many  demonstrations  and  experiments  described 
or  referred  to  can,  however,  easily  be  performed,  when  ad- 
visable, with  comparatively  little  trouble  or  expense. 


PKEFACE  V 

We  are  greatly  indebted  to  Professor  Werner  Spalteholz 
for  his  kind  permission  to  copy  certain  figures  from  his 
Hand  Atlas  of  Anatomy,1  a  book  which  we  recommend  as 
a  most  useful  reference  work  for  anatomical  study.  Our 
acknowledgments  are  also  due  to  Professor  Schottelius, 
from  whom  we  have  taken  many  of  the  figures  of  bacteria 
given  in  Part  II.  In  the  preparation  of  most  of  the  original 
figures  we  have  had  the  assistance  of  Dr.  Percy  G.  Stiles, 
whose  skill  as  a  draftsman,  combined  with  his  appreciation, 
as  a  physiologist,  of  our  point  of  view,  has  greatly  facili- 
tated this  part  of  our  work. 

The  index  has  been  prepared  with  special  reference  to 
its  use  as  a  glossary  of  anatomical  terms. 

1  Werner  Spalteholz,  Hand  Atlas  of  Human  Anatomy;  translated  by 
L.  F.  Barker.  G.  E.  Stechert,  New  York. 


CONTENTS 


PART   I.     PHYSIOLOGY 

CHAPTER  PAGE 

I.  THE  HUMAN  MECHANISM 3 

II.  THE  STRUCTURE  (ANATOMY)  OF  THE  HUMAN  MECH- 
ANISM      6 

III.  THE  FINER  STRUCTURE  OF  Two    TYPICAL  ORGANS, 

GLANDS  AND  MUSCLES.     THE  CONNECTIVE  TIS- 
SUES.    THE  LYMPHATIC  SYSTEM          ...  29 

IV.  THE  WORK   OF   THE    HUMAN  MECHANISM    THE   RE- 

SULTANT OF  THE  WORK  OF  ITS  DIFFERENT  OR- 
GANS AND  CELLS 44 

V.  WORK,  FATIGUE,  AND  RESTORATION  ....  55 

VI.  THE  INTERDEPENDENCE   OF  ORGANS  AND   OF  CELLS  65 
VII.  THE  ADJUSTMENT  OR  COORDINATION  OF  THE  WORK 

OF  ORGANS  AND  CELLS         .....  70 

VIII.  ALIMENTATION  AND  DIGESTION 88 

The   Supply  of   Matter   and   Power  to  the  Human 

Machine 88 

Digestion  in  the  Mouth.    The  Teeth.    Enzymes         .  98 

Digestion  in  the  Stomach 106 

Digestion  and  Absorption  in  the  Small  Intestine  and 

in  the  Large  Intestine 115 

IX.  THE  CIRCULATION  OF  THE  BLOOD      .         .         .         .132 

Blood  and  Lymph 132 

Mechanics  of  the  Circulation  of  the  Blood  and  of  the 

Flow  of  Lyrnph      .......  136 

The  Adjustment  of  the  Circulation  to  the  Needs  of 

Everyday  Life 149 

X.  RESPIRATION 162 

XL  EXCRETION 177 

XII.  THERMAL  PHENOMENA  OF  THE  BODY         .         .         .  187 

The  Constant  Temperature 187 

The  Regulation  of  the  Body  Temperature  .         .199 


viii  CONTENTS 

CHAPTER  *  PAGE 

XIII.  NUTRITION 211 

The  Sources  of  the  Power  and  Heat  of  the  Human 

Mechanism  ........  211 

Special  Effects  of  the  Different  Nutrients         .         .217 

Flesh,  Fat,  and  Glycogen 227 

The  Choice  of  Food  and  Nutrients           .         .         .  233 

XIV.  SENSE  ORGANS  AND  SENSATIONS      ....  244 
XV.  THE  NERVOUS  SYSTEM 266 

Its  Anatomical  Basis 266 

The  Physiology  of  the  Nervous  System    .         .         .273 


PART  II.     THE  HYGIENE  OF  THE  HUMAN  MECH- 
ANISM AND   THE   SANITATION  OF 
ITS  SURROUNDINGS 

XVI.   INTRODUCTORY 291 

Hygiene   the   Right   Use  and  Proper   Care   of  the 

Human  Mechanism 291 

Health  and  Disease 293 

The  Three  Great  Factors  of  Disease         .         .         .296 

PERSONAL  HYGIENE 

XVII.  MUSCULAR  ACTIVITY 304 

The  Ministry  of  Muscular  Activity  to  the  Body  as  a 

Whole 304 

General  Muscular  Exercise 314 

Muscular  Exercises  for  Special  Purposes.    Corrective 

Work.     The  Gymnasium 321 

XVIII.  THE    HYGIENE   OF   THE  NERVOUS    SYSTEM.      REST 

AND  SLEEP 334 

XIX.  THE  HYGIENE  OF  FEEDING 347 

XX.  FOOD  ACCESSORIES,  DRUGS,  ALCOHOL,  AND  TOBACCO  357 
XXI.  THE  PREVENTION  AND  CARE  OF  COLDS  AND  SOME 

OTHER  INFLAMMATIONS 380 

XXII.  THE  CARE  OF  THE  EYES  AND  EARS       .  395 


CONTENTS  ix 

CHAPTER  PAGE 

XXIII.  THE  HYGIENE  OF  THE  FEET          .     '   .         .         .  403 

XXIV.  BATHING 413 

XXV.  CLOTHING 418 

DOMESTIC  HYGIENE  AND  SANITATION 

XXVI.  THE  HOUSE  :  ITS  SITE,  CONSTRUCTION,  FURNISH- 
INGS, AND  CARE 425 

XXVII.  THE  WARMING  AND  LIGHTING  OF  THE  HOUSE    .  434 

XXVIII.   THE  AIR  SUPPLY  OF  THE  HOUSE.     VENTILATION  442 
XXIX.  THE  WATER  SUPPLY,  PLUMBING,  AND  DRAINAGE 

OF  THE  HOUSE.     GARBAGE  AND  RUBBISH     .  451 

PUBLIC  HYGIENE  AND  SANITATION 

XXX.  PUBLIC  HEALTH.     INFECTIOUS  AND  CONTAGIOUS 

DISEASES.     MICROBES 463 

XXXI.  SOME   MICROBIC  DISEASES  AND   THEIR  PREVEN- 
TION.   VACCINATION  AND  ANTITOXIC  SERUMS  477 
XXXII.  PUBLIC    SUPPLIES    OF    FOOD,   WATER,  AND   GAS. 

PUBLIC  SEWERAGE 505 

XXXIII.  THE  HYGIENE    AND  SANITATION  OF  TRAVELING, 

PUBLIC  CONVEYANCES,  PUBLIC  HOUSES,  ETC.  520 

XXXIV.  PUBLIC  PROTECTION  OF  THE  PUBLIC  HEALTH      .  529 
XXXV.  THE  HEALTH  OF  NATIONS 535 

INDEX  .  553 


THE  HUMAN  MECHANISM 

PAET  I 
PHYSIOLOGY 


CHAPTER  I 
THE  HUMAN  MECHANISM 

1.  The  Human  Body  a  Living  Organism.  —  The  human 
body,  as  compared  with  bodies  of  water  such  as  lakes  and 
seas,  or  with  heavenly  bodies  such  as  the  sun,  moon,  and 
stars,  is  a  small  mass  of  matter  weighing  on  the  average, 
when  fully  grown,  about  150  Ib.  and  measuring  in  length 
about  5  ft.  9  in.  It  is  neither  very  hot,  as  is  the  sun,  nor 
warm  in  summer  and  cold  in  winter,  as  are  many  bodies 
of  water,  but  in  life  and  health  has  always  almost  exactly 
the  same  moderate  temperature,  namely,  98.6°  F.  or  37.0°  C. 
The  human  body  is  not  homogeneous,  as  is  the  substance 
of  a  lake,  that  is  to  say,  alike  in  all  its  parts,  but  very 
unlike,  the  various  parts  —  eyes,  ears,  legs,  heart,  brain, 
muscles,  etc.  —  being  known  as  organs,  and  the  whole  body, 
therefore,  as  .the  human  organism. 

The  most  remarkable  peculiarity  of  the  human  body,  how- 
ever, is  that  it  is  a  living  organism.  A  watch  has  unlike 
parts  —  spring,  dial,  hands,  case,  etc.  —  which  are  essen- 
tially its  organs,  and  the  watch  might,  therefore,  be  called 
an  organism ;  yet  it  never  is  so  called.  We  speak  of  a 
well-organized  army,  navy,  government,  society,  church,  or 
school,  but  never  of  a  well-organized  automobile,  type- 
writer, printing  press,  or  locomotive,  —  apparently  for  the 
reason  that  in  army,  navy,  or  school  living  things  play  a 
principal  part,  while  in  mere  machinery  life  is  wholly  want- 
ing. The  highest  compliment  we  can  pay  to  a  machine  is 
to  say  that  it  seems  "  almost  alive,"  but  it  is  not  a  com- 
pliment to  any  human  being  to  describe  him  as  "  a  mere 

3 


THE  HUMAN  MECHANISM 

j."'  .'What  the  vital  property  is,  what  we  mean  by 
the  terms  "  life  "  and  "  living,"  no  one  can  exactly  tell. 
About  all  we  know  of  it  is  that  some  of  the  commonest  ele- 
ments of  matter  —  carbon,  hydrogen,  oxygen,  and  nitrogen, 
with  a  little  sulphur,  phosphorus,  and  a  few  other  elements 
—frequently  occur  combined  as  living  matter ;  and  that  this 
living  matter  has  marvelous  powers  of  growth,  repair,  and 
reproduction,  besides  a  certain  spontaneity,  originality,  and 
independence,  which  lifeless  matter  never  displays.  "  While 
there  is  life  there  is  hope  "  for  any  plant  or  any  animal,  but 
this  saying  does  not  apply  to  any  lifeless  machine,  however 
complex  or  wonderful. 

2.  The  Human  Body  a  Living  Machine  or  Mechanism. 
—  By  a  machine  we  mean  an  apparatus,  either  simple  or 
complex,  and  usually  composed  of  unlike  parts,  by  means 
of  which  power  received  in  one  form  is  given  out  or  applied 
in  some  other  form.  This  power  may  be  received,  for 
example,  in  the  form  of  heat,  or  electricity,  or  muscular 
effort,  or  as  the  potential  energy  of  fuel;  and  it  may  be 
given  out  as  heat,  or  electricity,  or  light,  or  sound,  or  as 
mechanical  work,  or  in  any  one  of  many  other  ways.  One 
of  the  simplest  of  all  machines  is  a  stove,  jin  apparatus 
composed  of  a  few  simple  parts  by  means  of  which  the 
potential  energy  or  power  of  fuel  —  wood,  coal,  gas,  or 
oil  —  is  liberated  and  applied  as  heat,  for  warming  or 
cooking.  A  lamp  is  a  still  simpler  machine  in  which  the 
potential  energy  or  power  of  gas  or  oil  is  liberated  and 
converted  into  useful  light.  A  candle  is  a  lamp  so  simple 
that  it  almost  ceases  to  be  a  machine,  and  yet  the  wick  is 
really  an  apparatus  for  securing  proper  combustion  of  wax 
or  tallow  to  provide  good  light. 

Machines  of  greater  complexity  are  watches  or  clocks, 
pieces  of  apparatus  composed  of  many  unlike  parts  which 
receive  power  in  comparatively  large  amounts  for  a  short 
time  during  the  process  of  winding,  store  it  as  potential 


THE  HUMAN  MECHANISM  5 

energy  in  coiled  springs  or  lifted  weights,  and  liberate  it 
slowly  in  the  mechanical  work  of  moving  the  hands  of  the 
timepiece  over  a  dial.  Still  more  complex  is  a  locomotive 
or  an  automobile,  machines  in  which  the  power  of  coal, 
oil,  gasoline,  or  other  fuel,  or  the  electricity  of  a  storage 
battery,  is  applied  to  swift  locomotion.  But  the  most  won- 
derful of  all  machines  is  the  human  body,  a  complicated 
piece  of  apparatus  in  which  the  power  stored  in  foods,  such 
as  starch,  sugar,  butter,  meat,  milk,  eggs,  and  fish,  is  trans- 
formed into  that  heat  by  which  the  body  is  warmed,  and 
into  that  muscular,  nervous,  digestive,  or  other  work  which 
it  performs. 

For  delicate  and  intricate  machinery  the  term  "  mechan- 
ism" is  often  employed,  and  we  may  therefore  describe 
the  human  body  either  as  the  "  human  organism,"  or  the 
"  human  machine,"  or,  perhaps  best  of  all,  as  the  HUMAN 

MECHANISM. 

The  study  and  the  science  of  the  construction  (structure) 
of  this  mechanism  is  called  its  anatomy ;  of  its  ordinary 
behavior,  operation,  or  working,  its  physiology;  of  its 
proper  management,  protection,  and  care,  its  hygiene.  This 
text-book  is  devoted  chiefly  to  an  account  of  its  operation 
and  care,  i.e.  to  its  physiology  and  hygiene ;  but  as  any 
true  comprehension  of  these  subjects  depends  upon  some 
preliminary  knowledge  of  the  parts  of  the  mechanism 
itself,  we  shall  begin  by  considering  briefly  the  structure 
or  anatomy  of  the  human  machine. 


CHAPTER  II 

THE   STRUCTURE   (ANATOMY)   OF   THE   HUMAN 
MECHANISM 

Anatomy  is  studied  partly  by  dissection,  which  reveals 
chiefly  those  organs  which  are  visible  to  the  naked  eye, 
and  partly  by  microscopic  examination,  which  gives  a  deeper 
insight  into  the  detailed  arrangement  of  the  cells  and  tissues 
of  which  the  organs  of  the  mechanism  are  composed.  The 
present  chapter  is  devoted  to  structures  or  organs  shown 
by  dissection,  —  the  gross  anatomy  of  the  body,  —  as  dis- 
tinguished from  its  microscopic  anatomy  (histology).1 

1  Further  explanation  of  the  structure  of  the  human  machine  will  be 
given  as  it  may  be  needed  in  subsequent  chapters.  At  this  point  it  is  of 
the  utmost  importance  that  the  student  thoroughly  master  the  general 
relations  of  the  more  important  organs  one  to  another;  this,  however, 
is  not  to  be  done  by  extensive  reading,  and  still  less  by  memorizing 
verbal  descriptions ;  the  aim  should  rather  be  to  acquire  from  figures  and 
diagrams,  or  better  yet  from  actual  dissection,  where  that  is  possible,  a  cor- 
rect mental  picture  of  the  structures  involved.  Far  more  can  be  learned 
by  constructing  drawings  or  diagrams  from  memory  than  by  the  mere 
memorizing  of  text.  The  drawings  may  lack  finish  and  may  be  at  first 
difficult  to  execute ;  but  so  long  as  they  represent  the  relations  of  the 
organs  one  to  another  they  accomplish  their  purpose  ;  beyond  this  point 
the  more  accurately  they  are  drawn  the  better. 

Moreover,  drawing  is  a  great  aid  to  dissection.  It  not  only  fixes  in  the 
memory  what  is  seen  but  it  compels  close  observation ;  when  one  draws  an 
object  he  is  forced  to  note  details  and  relations  of  structure  which  would 
otherwise  escape  observation.  Nor  is  the  freehand  drawing  which  is 
required  for  our  purpose  as  difficult  as  is  often  supposed  by  those  who 
have  never  seriously  used  it.  Let  the  student  attempt  to  reproduce  an 
object  from  his  memory  of  its  picture ;  begin  with  one  which  is  not  too 
complicated  (such  as  the  figure  of  the  peritoneum  and  mesentery  on 
page  14).  Where  he  does  not  know  how  to  represent  a  special  structure, 
let  him  refer  to  the  original  from  which  he  may  get  suggestions;  then 

6 


STEUCTUEE  OF  THE  MECHANISM  7 

The  human  mechanism  is  composed  of  different  parts, 
such  as  head,  neck,  trunk,  arms,  hands,  legs,  and  feet,  and 
each  of  these  in  its  turn  is  composed  of  lesser  parts.  Arms 
and  hands,  for  example,  are  covered  by  skin,  which  may  be 
moved  over  underlying  soft  parts  ;  at  the  ends  of  the 
fingers  the  place  of  the  skin  is  taken  by  nails,  while  scat- 
tered over  and  emerging  from  its  surface  are  hairs.  Through 
the  skin  may  be  seen  the  veins,  which  may  be  emptied  of 
the  purplish  blood  they  contain  by  pressing  one  finger  on 
a  part  of  the  vein  near  the  finger,  and  pushing  another 
finger  along  the  vein  toward  the  wrist ;  so  long  as  pressure 
is  maintained  by  both  fingers  the  vein  remains  collapsed, 
but  on  removing  the  first  finger  it  fills  again  with  blood. 
Finally,  through  the  soft  parts  (flesh)  may  be  felt  the  hard 
bones.  In  general  these  various  parts  of  which  the  body  is 
composed  are  known  as  its  organs,  and  because  it  possesses 
organs  it  is  called  an  organism  (p.  3). 

1.  The  Skin. —  The  body  is  everywhere  covered  by  a 
complex  protective  and  sensitive  organ,  the  skin.     Only 
the  eyes  and  nails  seem  to  be  exceptions ;  but  as  a  matter 
of  fact  the  exposed  surface  of  the   eye  is  covered  by  a 
very  thin,  transparent  portion  of  the  skin,  and  the  nails 
are  really  modified  portions  of  skin. 

2.  Subcutaneous  Connective  Tissue. — On  cutting  through 
the  skin  we  find  that  it  is  bound  to  the  underlying  flesh 
(chiefly  meat  or  muscle)  by  what  is  known  as  connective 
tissue,  the  structure  of  which  we  shall  study  in  the  next 
chapter.    Meanwhile  we  may  notice  that  it  contains  blood 
vessels,  that  at  some  places  it  is  more  easily  stretched  than 

close  the  book  and  draw  from  memory ;  any  completed  part  of  the  work 
may  be  compared  with  the  original  and  possible  improvements  discovered. 
Such  practice  may  well  precede  drawing  from  an  actual  dissection  and 
will  pave  the  way  to  the  latter.  At  all  events  let  the  student  understand 
thoroughly  that  in  the  present  chapter  the  figures,  supplemented  if  pos- 
sible by  actual  dissections,  form  the  main  objects  of  study ;  the  text  is 
strictly  subordinate  to  the  figures. 


8  THE  HUMAN  MECHANISM 

at  others,  and  that  when  a  flap  of  skin  is  pulled  away 
from  the  muscles,  this  subcutaneous  tissue  fills  with  air. 
It  often  contains  large  quantities  of  fat. 

3.  Muscles  and  Deeper  Connective  Tissues.  —  The  sub- 
cutaneous connective  tissue  sometimes  connects  or  binds 
the  skin  directly  to  bone,  as  in  parts  of  the  head ;  usually, 
however,  in  the  neck,  trunk,  and  limbs  the  underlying 
tissue  is  the  red  flesh,  or  muscle,  familiar  to  us  as  "  lean  of 
meat."  If  the  skin  be  removed  from  the  forearm,  it  at  once 
becomes  evident  that  this  mass  of  meat  or  flesh  is  composed 
of  a  number  of  muscles  which  may  be  separated  from  one 
another  more  or  less   completely.    In   doing  this  it  will 
be  found  that  the  muscles  are  held  together  by  connective 
tissue  in  most  respects  quite  similar  to  that  immediately 
under  the  skin.     Further  dissection  will  show  that  one  or 
another  form  of  this  tissue  is  the  means  of  binding  other 
organs  together;  thus  the  muscles  are  joined  to  the  bones 
by  a  very  dense,  compact,  and  strong  form  known  as  tendon; 
the  bones  are  united  by  a  somewhat  similar  form  known 
as  ligament;  and  so  on.    The  physical  characters  of  the 
tissue  differ  widely,  according  to  its  situation  and  the  use 
subserved ;  but  one  form  shades  more  or  less  into  another, 
and  we  have  no  difficulty  in  recognizing  the  general  simi- 
larity which  leads  us  to  group  them  all  together  in  one  class. 

4.  Muscles  attached  to  Bones.  — When  a  muscle  is  care- 
fully dissected  away  from  neighboring  muscles  and  other 
organs,  it  is  almost  always  found  that  it  is  attached  to  one 
and  usually  to  two  bones ;  this  union  is  frequently  made 
by  means  of  a  tendon,  as  in  the  case  of  the  large  muscle  of 
the  calf  of  the  leg,  which  is  attached  at  one  end  to  the 
bone  of  the  thigh  and  at  the  other  to  that  of  the  heel.    A 
good  example  of  the  direct  attachment  of  muscles  to  bones 
is  furnished  by  those  muscles  which  lie  between  the  ribs 
(see  Fig.  141).  In  either  case  the  shortening  of  the  muscle 
brings  closer  together  the  bones  to  which  it  is  attached. 


STRUCTURE  OF  THE  MECHANISM 


9 


5.  Definition  of  Some  Anatomical  Terms.  —  Before  pro- 
ceeding further  we  must  agree  upon  the  exact  meaning  of 
certain  anatomical  terms.  We  often  speak  of  one  part 
of  the  body  as  being  "  above  "  or  "  below,"  "  before  "  or 
"behind,"  another.  Such  terms,  however,  are  confusing, 
because  their  meaning  depends  upon  the  position  of  the 
body  at  the  time  they  are  used.  For  example,  when  one 
is  lying  on  his  back  the  head  is  in  front  of,  or  before,  the 
trunk  ;  but  when  he 
is  standing  on  his 
feet  it  is  above  the 
trunk. 

Now  the  body  is 
certainly  divided 
into  right  and  left 
halves,  which  are 
much  alike  exter- 
nally, though  this 
likeness  is  not  so 
marked  in  the  inter- 
nal parts.  Right  and 
left  then  have  their 

ordinary    meanings, 

FIG.  1.  Cross  section  of  arm 


skin  ;  B,  subcutaneous  connective  tissue,  bind- 
ing the  skin  to  the  muscles  D,  and  continuous 
with  the  connective  tissue  which  binds  together 
the  muscles  ;  (7,  blood  vessels  and  nerves 


and  that  without  re- 
gard to  the  various 
positions  the  body 
may  take. 

To   indicate   that 

any  part  is  nearer  the  head  than  another  part,  we  say  that 
the  former  is  anterior  to  the  latter;  to  indicate  that  the 
latter  is  further  away  from  the  head,  we  say  it  is  posterior 
to  the  former. 

Finally,  the  region  popularly  known  as  the  back  is  called 
dorsal  (Latin,  dorsum,  back),  that  opposite  the  back  being 
called  ventral  (Latin,  venter,  belly).  Thus  the  nose  is  on 


10 


THE  HUMAN  MECHANISM 


the  ventral  side  of  the  head ;  the  toes  are  at  the  posterior 
extremity  of  the  foot. 

6.  The  Body  Cavities.  —  There  is  one  striking  and  im- 
portant structural  difference  between  the  trunk  and  the 

limbs ;  the  former 
contains  a  central 
body  cavity,  com- 
pletely filled,  how- 
ever, with  various 

Lung  \  \  organs,   while    the 

arms  and  legs  are 
each  composed  of 
a  continuous  mass 
of  tissues,  viz.  mus- 
cle, blood  vessels, 
nerves,  bone,  etc., 
all  bound  together 
by  connective  tis- 
sue. 

The  cavity  of  the 
trunk,  or  body  cav- 
ity, is  subdivided 
transversely  by  the 
dome-shaped  mus- 
cle known  as  the 
FIG.  2.  The  thoracic  or  pleural,  and  the  abdomi-  7 .  7  •  , 

'   nal  or  peritoneal,  cavities  filled  with  organs       diaphragm  into  two 

cavities,  —  an  an- 
terior, known  as  the  thoracic  or  pleural  cavity,  and  a  poste- 
rior, known  as  the  abdominal  or  peritoneal  cavity.  Both 
cavities  are  lined  by  a  thin,  smooth,  shiny  membrane,  that 
of  the  thoracic  being  known  as  the  pleura,  and  that  of  the 
abdominal  as  the  peritoneum. 

Filling  the  pleural  cavity  are  found  the  heart,  lungs, 
oesophagus,  windpipe  or  trachea,  and  many  great  blood  ves- 
sels; filling  the  abdominal  cavity,  the  stomach,  the  small 


STRUCTURE  OF  THE  MECHANISM 


11 


intestine,  the  large  intestine,  the  liver,  the  pancreas,  the  kid- 
neys, the  spleen,  and  other  organs  together  with  numerous 
large  and  important  arteries  and  veins.  In  both  cavities 
the  lining  membrane  (pleura  or  peritoneum)  is  folded  back 
over  the  organs ;  that  is  to  say,  the  organs  do  not  really 
lie  in  the  cavities,  but  only  fill  them  as  the  hand  would 
fill  a  bladder  one  wall 
of  which  it  pushes  in 
against  the  other.  The 
surfaces  of  the  organs, 
like  the  walls  of  the  cav- 
ity, are  consequently 
smoothly  covered  and 
glide  over  one  another 
with  very  little  friction. 
The  preservation  of 
these  pleural  and  peri- 
toneal linings  in  their 
normal  condition  is  a 
matter  of  great  impor- 
tance ;  when  inflamed 
or  otherwise  injured 
their  surfaces  become 
roughened,  and  adhe- 
sions of  connective  tis- 
sue often  develop 
between  them  which 

fasten  the  organs  together,  or  to  the  walls  of  the  cavity, 
so  that  surgical  interference  is  sometimes  necessary.  Pleu- 
risy is  such  an  inflammation  of  the  pleura,  peritonitis  of 
the  peritoneum ;  and  both  are  very  serious  conditions. 

7.  Attachment  of  the  Organs  to  the  Walls  of  the  Pleu- 
ral and  Peritoneal  Cavities.  —  The  pleural  cavity  is  com- 
pletely divided  by  a  median  partition  of  connective  tissue 
(the  mediastinum),  within  which  are  found  the  trachea,  the 


FIG.  3.  Cross  section  of  the  thorax  anterior 
to  the  bifurcation  of  the  trachea 

-4,  a  vertebra  of  the  spinal  column;  B,  spinal 
cord ;  (7,  the  pleural  cavity  (which  is  exag- 
gerated for  the  sake  of  clearness),  the  sur- 
face of  the  lung  being  actually  in  contact 
with  the  body  wall.  The  oesophagus,  tra- 
chea, together  with  several  large  arteries 
and  veins  are  shown  in  the  mediastinum 
ventral  to  the  vertebra,  and  in  the  order 
named 


12  THE  HUMAN  MECHANISM 

oesophagus,  the  great  blood  vessels,  and —  lying  within  a 
special  cavity  of  its  own  —  the  heart.  Approximately 
halfway  from  the  anterior  to  the  posterior  border  of  the 
mediastinum  the  trachea  divides  within  that  membrane 
into  two  tubes  or  bronchi,  which  pass  through  the  mediasti- 
num outward  to  the  right  and  left  lung  respectively.  The 


FIG.  4.  Cross  section  of  thorax  posterior  to  bifurcation  of  trachea 
A,  bronchus,  entering  the  lung;  B,  the  aorta  cut  at  its  origin  and  again  at 
the  descending  part  of  its  arch ;   (7,  the  pericardial  space ;   D,  the  pleural 
cavity;    P. A.,  the  pulmonary  artery 

pleural  lining  of  the  mediastinum  is  pushed  outward  by 
these  tubes,  and,  as  they  end  in  the  lungs,  forms  the  pleural 
covering  of  the  latter  (Fig.  5).  Consequently  the  organs 
of  the  pleural  cavity  either  lie  within  the  mediastinum 
(heart,  oesophagus,  trachea,  etc.)  or  else  are  covered  by 
extensions  of  the  mediastinal  pleura  (bronchi  and  lungs). 

The  abdominal  cavity  is  not  similarly  separated  into  right 
and  left  halves ;  but  a  membrane,  the  mesentery,  passes 
ventrally  from  the  dorsal  wall  to  the  stomach  and  intes- 
tine, which  are  slung  in  it  somewhat  as  a  man  lies  in  a 
hammock.  The  line  of  attachment  of  this  mesentery  to  the 


STRUCTURE  OF  THE  MECHANISM 


13 


small  intestine  is  much  longer  than  that  of  its  attachment 
to  the  body  wall ;  hence^  it  has  the  general  shape  of  a  ruffle, 
or  flounce, — an  arrangement  which  permits  the  suspension 
of  the  very  long  intestine  (20  to  25  ft.)  from  the  compara- 
tively short  median 
dorsal  body  wall 
(see  Fig.  136).  The 
great  arteries  and 
veins  lie  in  the 
mesentery  near  the 
dorsal  body  wall 
and  branches  are 
distributed  from 
them  to  the  intes- 
tine within  this 
expanding  mem- 
brane (see  Fig.  143). 
The  kidneys  do 
not  lie  movably 
suspended  in  the 
abdominal  cavity, 
as  do  the  intes- 
tines, but  are 
large  organs,  one 
on  each  side,  sit- 
uated near  the 
spinal  column 
and  dorsal  to  the 
abdominal  cavity 


5.  Diagrammatic  vertical  right-to- 
left  section  of  the  right  thorax 

A,  muscles,  ribs,  etc.,  of  the  body  wall ;  B, 
pleura,  lining  the  same ;  (7,  the  pleural 
space  or  cavity ;  D,  the  pleural  covering 
of  the  lung ;  E,  connective  tissue  of  the 
lung;  F,  alveoli  of  the  lung;  G,  dia- 
phragm ;  H,  trachea ;  I,  right  bronchus, 
branching ;  K,  the  pericardial  space  in 
which  lies  the  heart.  Note  the  division 
of  the  lung  into  two  lobes 


from  which  they  are  sep- 
arated by  the  peritoneum. 
Arteries  and  veins  are  sup- 
plied to  them  from  the  large  median  artery  and  the  median 
vein  already  referred  to  (aorta  and  vena  cava,  Fig.  15), 
and  these  renal  arteries  and  veins  are  likewise  outside  the 
abdominal  cavity. 


14 


THE  HUMAN  MECHANISM 


The  relation  of  the  other  organs  to  the  peritoneum  is  more 
complicated,  notably  in  the  case  of  the  liver;  but  in  all  cases 

the  organs  are  in- 
closed, or  wrapped, 
either  in  a  fold  of 
the  peritoneum,  as 
is  the  kidney,  or  in 
a  fold  of  the  mes- 
entery, as  is  the  in- 
testine; and  their 
blood  and  nerve  sup- 
plies run  to  them 
in  similar  folds. 
FIG.  6.  Diagrammatic  cross  section  of  the  ab-  g  The  Axial 

dominal  cavity 

Showing  the  relation  of  the  kidneys  and  great  blood 
vessels  to  the  peritoneum.     The  intestine  has 


Skeleton.  — The 
bones   and    carti- 


been  removed,  the  cut  border  of  the  mesentery     ,  Q£ 

being  shown  ° 

skeleton  is  com- 
posed may  be  classified  into  an  axial  skeleton  (of  the  head, 
neck,  and  trunk)  and  an  appendicular  skeleton  (of  the  arms 
and  legs).  The  axial  skeleton  comprises  (1)  the  backbone 
or  vertebral  column,  (2)  the  ribs  and  breastbone,  and 
(3)  the  skull. 

9.  The  Backbone  or  Vertebral  (Spinal)  Column.  —  This  is 
composed  of  separate  irregular  ringlike  bones  or  vertebra 
placed  one  above  another,  and  bound  together  by  bands 
of  strong  connective  tissue  known  as  ligaments.  It  is  cus- 
tomary to  divide  the  backbone  into  the  following  regions. 

a.  Cervical.  7  vertebrae  of  the  neck. 

b.  Thoracic.  12  vertebra^  of  the  chest,  to  which  ribs  are  attached. 

c.  Lumbar.       5  vertebrae  of  the  "small  of  the  back." 

d.  Sacral.        5  vertebrae  (fused  together)  to  which  the  large 

hip  bones  are  attached. 

e.  Coccygeal.  4  or  5  very  small  simple  vertebrae  (constituting 

the  skeleton  of  a  rudimentary  tail  and  corre- 
sponding to  the  tail  of  lower  animals). 


FIG.  7.  The  skeleton  entire 
15 


16 


THE  HUMAN  MECHANISM 


FIG.  8.  Sixth  thoracic 

vertebra 
Seen  from  above 


When  one  looks  at  the  spinal  column  from  behind,  the 
vertebrae  are  seen  to  be  placed  one  upon  another,  but  all 

in  the  median  dorsoventral  plane 
of  the  body  (see  Fig.  7).  Seen  from 
the  side,  however,  several  curves 
come  into  view,  as  shown  in  Fig.  1 0. 
On  the  ventral  side,  in  the  cervi- 
cal and  upper  thoracic  region,  the 
curvature  is  slightly  convex,  in 
the  thoracic  region  it  is  quite  con- 
cave, in  the  lumbar  region  slightly 
convex,  and  in  the  sacral-coccygeal 
region  again  concave.  It  may  well 
be  asked  how  these  separate  verte- 
brae, piled,  as  it  were,  one  above  another,  maintain  their 
proper  relative  positions.  This  is  partly  due  to  the  shape 
of  the  individual  vertebrae,  partly  to  the  ligaments  (p.  17) 
which  pass  from  one  vertebra  to 
another  and  limit  the  movements 
of  each,  and  partly  to  the  action 
of  muscles  which  are  placed  upon 
opposite  sides  of  the  vertebrae 
and  by  their  antagonistic  action 
hold  them  in  place.  The  action 
of  muscles  and  ligaments  upon 
the  bones  may  be  illustrated  by 
two  blocks  of  wood  held  to- 
gether by  two  rubber  bands  (m,m', 
Fig.  11)  slightly  stretched ;  so 
long  as  each  pair  of  opposite 
bands  pulls  with  the  same  force, 
the  blocks  are  kept  in  what  we  may  call  their  resting 
position.  Here  the  rubber  bands  represent  two  of  the 
antagonistic  muscles,  which,  by  maintaining  a  steady  and 
equal  pull  on  the  opposite  sides  of  the  vertebrae,  keep 


FIG.  9.  Sixth  thoracic 

vertebra 
Seen  from  the  side 


STRUCTURE  OF  THE  MECHANISM 


17 


Cervical 


Thoracic 
(or  Dorsal) 


them  in  place.  Should  one  pull  harder  than  its  antago- 
nist, as  when  a  muscle  contracts  (see  Chapter  IV),  the 
antagonist  will  be  stretched  and  the  bones  become  inclined 
somewhat  toward  one  another,  as 
shown  in  the  figure. 

This  principle  of  muscular  an- 
tagonism is  quite  general  in  the 
maintenance  of  the  proper  rela- 
tive positions  of  bones  in  the 
body.  Almost  every  joint  is  the 
theater  of  such  plays  of  antago- 
nistic muscles,  which  serve  the 
double  function  of  keeping  the 
bones  in  proper  position  with 
regard  to  one  another,  and  of 
producing  movement  at  the  joint, 
the  amount  of  this  movement 
being  limited  by  the  slack  but 
inextensible  connective-tissue 
ligaments  which  bind  the  bones 
together.  In  Figure  11  both  the 
shortening  of  the  muscle  and  the 
slackness  of  the  ligaments  is  pur- 
posely exaggerated,  in  order  to 
represent  more  clearly  the  func- 
tions of  these  tissues.  Ligaments 
may  also  guide  the  movement  of 
bones  by  preventing  motion  in 
one  direction  or  another. 

10.  The  Ribs.  —  Each  rib  con- 
sists of  a  bony  and  a  cartilaginous  portion.  The  former 
articulates  (i.e.  forms  a  joint  with)  the  vertebral  column, 
while  the  latter  continues  this  bony  portion  to  the  ventral 
median  breastbone,  to  which  it  is  directly  joined.  The  ribs 
form  the  framework  for  the  thorax  and  may  be  lifted  or 


Lumbar 


Sacral 


FIG.  10.  The  vertebral  col- 
umn 
Seen  from  the  side 


18 


THE  HUMAN  MECHANISM 


FIG.  11 

Model  showing  the  action  of  muscles  on  two  vertebrae,  and  of  the  ligaments 
(I,  I')  in  limiting  the  amount  of  movement.  The  contraction  of  the  muscle  m 
stretches  its  antagonist  m'.  The  amount  of  movement  is  greatly  exaggerated 

lowered  by  muscles  which  connect  them  with  the  vertebral 

column   and   other   parts  of  the 
skeleton  (see  Fig.  12). 

11.  The  Skeleton  and  the  Cen- 
tral Nervous  System.  —  The  skull 
consists  of  the  bones  of  the  face 
and  those  of  the  cranium,  the  lat- 
ter holding  the  brain.  It  is  sup- 
ported on  the  spinal  or  vertebral 
column  whose  ringlike  vertebrse 
inclose  a  bony  canal  continuous 
with  the  cranial  cavity.  This  is 
known  as  the  spinal  or  vertebral 
canal  in  which  lies  the  spinal  cord,1 
—  the  continuation  of  the  central 

nervous  system  posterior  to  the  brain. 


FIG.  12.  Dorsal  view  of  ver- 
tebrae and  ribs 

Showing  some  of  the  muscles 
which  lift  or  raise  the  ribs 


1  The  terms  "spinal  cord,"  "spinal  column,"  and  "  spinal  canal "  are 
sometimes  confused  by  beginners.  The  spinal  column  is  the  entire  bony 
framework  formed  by  the  vertebrae,  —  the  whole  backbone  ;  it  surrounds 
the  spinal  canal  which,  in  turn,  contains  that  part  of  the  nervous  system 
known  as  the  spinal  cord. 


STRUCTURE  OF  THE  MECHANISM 


19 


Nerves,  which  pass  through  small  openings  in  the  cra- 
nium   and    between    the 

vertebrae,  leave  the  brain  ifStitP&^\  Cranium 

and  cord  and  end  in  the 
muscles,  skin,  glands,  and 
other  organs  of  the  body 
(see  Chapter  VII). 

12.  The  Appendicular 
Skeleton. —  The  bones  of 
the  arm,  leg,  hand,  and 
foot  may  readily  be  felt, 
and  are  sufficiently  fa- 
miliar. We  may,  however, 
call  attention  to  the  simi- 
larity in  the  number  and 
form  of  the  bones  of  the 
arms  and  legs,  a  similarity 
which  is  not  only  helpful 
in  mastering  their  names 
and  arrangement,  but  is 
also  suggestive  of  the 
similarity  of  function  in 
quadrupeds,  both  limbs  in 
these  animals  being  or- 
gans of  locomotion. 


Thorax 


Vertebral 
Canal 


Sacrum 


Pelvis 


ARM 

Humerus,  single  long  bone  of  the 
upper  arm. 

Radius  and  ulna,  two  nearly  par- 
allel bones  of  the  forearm. 

Eight  small  irregular  bones  of 
the  wrist. 

Five  parallel  bones  of  the  palm. 

,  f  Thumb,  two  bones. 
Bones  of     ^A. 

4  Other  fingers, 
fingers  ,         * 

three  bones. 


FIG.  13.  Median  dorsoventral  section 
of  the  skeleton 

LEG 

Femur,  single  long  bone  of  the 

thigh. 
Tibia    and   fbula,     two    nearly 

parallel  bones  of  the  lower  leg. 
Seven  small  irregular  bones   of 

the  ankle  and  heel. 
Five  parallel  bones  of  the  instep. 

,  f  Great  toe,  two  bones. 
Bones  of     _ ,, 

toes         Other  toes, 

three  bones. 


20 


THE  HUMAN  MECHANISM 


The  legs  are  attached  to  the  vertebral  column  by  the 
large  hip  bones,  which  articulate  directly  and  immovably 
with  the  sacrum l ;  but  the  humerus,  or  bone  of  the  upper 
arm,  articulates  on  each  side  with  one  of  a  pair  of  bones 
which  form  the  shoulder  girdle,  or  skeleton  of  the  shoulder 
region ;  this  pair  consists  of  the 
collar  bone  (clavicle)  ventrally  and 
the  shoulder  blade  (scapula)  dorsal ly. 
The  clavicle  articulates  with  the 
head  of  the  breastbone  ;  otherwise 
the  shoulder  girdle,  with  the  arm 
attached  to  it,  is  connected  with 
the  axial  skeleton  by  muscles  only. 
A  wide  range  of  movement  is  thus 
secured  at  the  shoulder  joint. 

13.   Organs   of   Digestion.  —  The 
digestive  system  consists  essentially 
of  a  long  tube,  the  alimentary  canal, 
passing   through   the   body.2     Into 
this   tube  at  various   points   ducts 
FIG.  14.  Diagrammatic   from  a  number  of  glands  pour  di- 
median     dorso  ventral          ,  •       •    •  rr<-i        T 

section  of  the  nasal  and  $estwe  Juwes'    The  Alimentary  canal 
throat  passages  begins  with  the  mouth  cavity  and  its 

C,  nasal  cavities ;  M,  mouth   familiar  organs,  the  teeth,  the  tongue, 
cavity;    r,  tongue;  E,   etc      this  cavity  opens  posteriorly 

epiglottis;   G,  glottis,  or  J       * 

opening  from  the  pharynx  into  that  of  the  pharynx,  into  which 
into  the  trachea;  u,  the  a|go  Opens  the  nasal  cavity,  separated 

end  of  the  soft  palate ;  0,  r 

cEsophagus  from  the  mouth  only  by  the  palate 

(see  Fig.  14). 

On  the  ventral  side  of  the  phary^nx  just  beyond  the 
root  of  the  tongue  is  the  slitlike  opening  of  the  windpipe 
(see  Section  14);  posteriorly  the  pharynx  is  continued  in  the 
long  gullet,  or  oesophagus,  a  tube  which  passes  downward 

1  The  sacrum  and  the  two  hip  bones  together  form  the  pelvis. 

2  See  Fig.  135  for  the  general  arrangement  of  the  organs  of  digestion. 


STRUCTURE  OF  THE  MECHANISM  21 

through  the  neck  and  thorax  (within  the  mediastinum)  to 
join  the  stomach,  which  it  enters  immediately  after  passing 
through  the  diaphragm. 

The  stomach  is  a  large  pouch,  with  contractile  walls  per- 
mitting adaptation  of  its  size  to  the  bulk  of  food  it  may 
contain.  Its  situation  is  shown  in  Fig.  135,  which  also 
shows  how  it  opens  on  the  right  side  of  the  body  into  the 
very  long,  coiled,  small  intestine.  The  coils  of  this  part  of 
the  tube  may  be  followed  for  from  twenty  to  twenty-four 
feet,  to  the  large  intestine,  into  one  side  of  which  it  opens. 
The  large  intestine,  or  colon,  consists  of  three  portions:  the 
first  ascending  on  the  right  side  to  the  general  level  of  the 
stomach,  the  second  passing  transversely  at  this  level  from 
right  to  left,  and  the  third  descending  on  the  left  side  to  the 
rectum,  the  posterior  terminal  portion  of  the  digestive  tube. 

Numerous  glands  pour  secretions  through  ducts  into 
the  digestive  tube,  the  following  being  the  more  important 
with  their  places  of  discharge :  salivary  glands  (see  Chap- 
ter III)  —  mouth  ;"  liver  —  beginning  of  small  intestine ; 
pancreas  —  beginning  of  small  intestine  (see  Fig.  50). 
Smaller  glands  empty  into  the  stomach  and  intestines  at 
numerous  places. 

14.  The  Organs  of  Respiration.  —  The  organs  of  respira- 
tion consist  of  the  right  and  left  lungs  (see  Fig.  5)  from 
each  of  which  a  single  bronchus  (pi.,  bronchi)  leads  to  the 
trachea,  or  windpipe.  The  walls  of  the  trachea  and  bronchi 
are  kept  from  collapsing  by  successive  rings  of  cartilage. 
Anteriorly  the  trachea  opens  into  the  pharynx  through  the 
larynx,  or  voice  box,  the  cartilages  of  which  may  be  felt  in 
the  throat  at  the  root  of  the  tongue.  The  familiar  hoarse- 
ness which  accompanies  inflammatory  roughening  of  the 
lining  of  the  larynx  shows  how  important  is  this  organ  in 
the  production  of  the  voice.  The  respiratory  and  digestive 
paths  cross  in  the  pharynx,  the  former  reaching  the  exterior 
through  the  nose,  the  latter  through  the  mouth. 


22  THE  HUMAN  MECHANISM 

15.  The  Organs  of  Circulation.  —  The  position  of  the 
heart  and  the  great  blood  vessels  in  the  thorax  has  been 
described  on  page  11.  The  heart  is  essentially  a  large  mass 
of  muscle,  containing  a  cavity  which  is  divided  into  right 
and  left  halves,  wholly  separate  from  each  other.  The 
cavity  on  each  side  is  divided  into  that  of  the  large  ventricle 
with  very  thick  walls,  and  that  of  the  much  smaller  auricle. 
The  heart  is  thus  composed  of  right  and  left  auricles  and 
right  and  left  ventricles.  Valves  are  so  placed  in  the 
heart  as  to  allow  blood  to  flow  in  one  direction  only  (see 
Fig.  64). 

The  arteries  are  tubes  which  carry  the  blood  to  the 
tissues,  and  from  each  side  of  the  heart  a  single  artery 
takes  its  origin,  —  the  pulmonary  artery  from  the  right 
ventricle  and  the  aorta  from  the  left  ventricle.  The  pul- 
monary artery  supplies  the  lungs  with  blood,  while  all 
other  organs  are  supplied  by  the  aorta. 

The  veins  are  tubes  which  conduct  the  blood  from  the 
various  organs  to  the  heart.  Beginning  in  the  tissues  as 
microscopic  tubes,  they  unite  to  form  larger  and  larger 
tubes  as  they  approach  the  heart;  those  visible  through 
the  skin  of  the  hand  may  be  regarded  as  of  medium  size ; 
as  the  union  goes  on,  the  size  of  the  vessels  increases  until 
finally  at  the  heart  there  are  only  two  great  veins  on  the 
right  side  (superior  vena  cava  and  inferior  vena  cava)  and 
four  on  the  left  (pulmonary  veins).  The  venae  cav£e  bring 
blood  back  from  those  portions  of  the  body  which  are  sup- 
plied by  the  aorta,  that  is  to  say,  from  all  parts  of  the 
body  except  the  lungs;  the  pulmonary  veins  bring  blood 
back  only  from  the  lungs,  that  is  to  say,  from  the  organs 
supplied  by  the  pulmonary  arteries.  The  vense  cavse  empty 
into  the  right  auricle,  the  pulmonary  veins  into  the  left 
auricle.  The  general  arrangement  of  heart,  arteries,  and 
veins  is  shown  in  Fig.  15,  and  the  figures  in  Chapter  IX 
(especially  65  and  66)  should  also  be  consulted. 


STKUCTUKE  OF  THE  MECHANISM  23 

The  blood  flows  in  the  following  circuit. 
Right  ventricle  to 


Pulmonary  circulation  ^ 


Pulmonary  artery  to 


Lungs  to 

Pulmonary  veins  to 
f  Left  auricle  to 
Left  ventricle  to 
Aorta  and  its  branches  to 

Systemic  circulation      .  All  organs  of  the  body  (except  the  lungs)  to 
Veins  which  unite  to  form  the  venge  cavse  to 
Right  auricle  to 
Right  ventricle. 

Thus  the  blood  which  leaves  the  left  ventricle  flows  to 
the  different  organs  of  the  body  (except  the  lungs),  and 
returns  by  way  of  the  veins  to  the  right  side  of  the  heart ; 
thence  it  passes  through  the  lungs  and  again  to  the  left 
auricle  and  ventricle,  thus  completing  the  "circulation." 
The  term  "  circulation,"  strictly  speaking,  is  applied  to  the 
entire  circuit  which  the  blood  must  traverse  until  it  returns 
again  to  the  point  from  which  it  started ;  it  is  often  con- 
venient, however,  to  use  it  to  denote  the  course  from  the 
right  ventricle  to  the  left  auricle,  or  from  the  left  ventricle 
to  the  right  auricle  ;  in  this  case  we  speak  of  the  former  as 
the  pulmonary  and  of  the  latter  as  the  systemic  or  aortic  cir- 
culation. In  this  sense  there  may  be  said  to  be  a  "  double  " 
circulation. 

The  veins  have  thinner  walls  than  the  corresponding 
arteries,  and  those  of  the  systemic  circulation  contain  pur- 
plish or  even  bluish  blood,  while  the  arteries  of  the  same 
circulation  contain  bright  scarlet  blood.  The  bright  color 
of  the  arterial  blood  is  due  to  the  fact  that  it  contains 
more  oxygen.  The  change  from  purple  to  scarlet  occurs 
in  the  lungs,  and  the  reverse  change  in  the  organs  sup- 
plied by  branches  of  the  aorta.  Consequently  the  blood  of 
the  pulmonary  arteries  is  blue  or  venous  in  color,  and  that 
of  the  pulmonary  vein  scarlet  or  arterial. 


FIG.  15.  Diagram  of  the  circulation  of  blood 

R.A.,  right  auricle;  L.A.,  left  auricle;  R.V.,  right  ventricle;  L.V.,  left 
ventricle;  P. A.,  pulmonary  artery;  A,  pulmonary  artery  and  vein  of 
right  lung;  .B,  pulmonary  artery  and  vein  of  left  lung;  C,  carotid 
artery  to  head,  showing  branch  of  left  subclavjan  artery;  D,  portal 
vein;  E,  hepatic  vein;  F,  hepatic  artery;  G,  jugular  vein,  bringing 
blood  from  head  and  neck 

24 


STEUCTUEE  OF  THE  MECHANISM 


25 


16.  The  Course  and  Branches  of  the  Pulmonary  Artery 
and  Vein.  —  Soon  after  leaving  the  right  ventricle  the 
pulmonary  artery  divides  into  two  branches,  one  going  to 
each  lung.    Each  of  these  further  divides  as  it  plunges 
into  the  substance  of  the  lung  alongside  the  bronchus. 
The  course  of  the  four  pulmonary  veins  may  be  similarly 
traced  into  the  lungs,  from  which  they  bring  the  blood 
back  to  the  heart  (Fig.  15). 

17.  The  Course  and  Branches  of  the  Aorta.  —  The  aorta 
passes  anteriorly  from   the  left  ventricle,  but  very  soon 
arches   dorsally  and 

posteriorly,  forming 
the  arch  of  the  aorta 
(Fig.  15);  the  gen- 
eral course  of  the 
artery  can  be  best 
understood  from  the 
figures,  or  from  ac- 
tual dissection.  The 
arch  of  the  aorta  is 
continued  in  the 
large  dorsal  aorta 
which  passes  poste- 
riorly on  the  left 
side  of  the  mediasti- 
num near  the  spine, 
through  the  dia- 
phragm, to  the  lower 
portion  of  the  abdominal  cavity,  where  it  divides  into  two 
large  arteries  which  supply  blood  to  the  hips  and  legs. 
From  the  arch  of  the  aorta  three  large  arteries  pass  to  the 
head,  neck,  shoulders,  and  arms ;  from  the  thoracic  dorsal 
aorta  arise  a  number  of  small  arteries  which  supply  the 
muscles  and  other  organs  of  the  thoracic  wall ;  immediately 
after  passing  through  the  diaphragm  two  large  branches 


FIG.  16.  A  network  of  capillaries,  with  the 
artery  a  and  vein  v  (highly  magnified) 


26 


THE  HUMAN  MECHANISM 


go  to  the  stomach,  spleen,  liver,  pancreas,  and  a  large  part 
of  the  small  intestine;  posterior  to  these  the  renal  arteries 
pass  right  and  left  to  the  kidneys,  and  still  further  down 
a  large  artery  supplies  the  lower  small  intestine  and  the 
large  intestine.  The  supply  to  the  legs  has  already  been 
mentioned.  Other  small  arteries  arise  from  the  abdominal 

aorta  and  are  distributed 
to  the  muscles  and  skin  of 
the  back.  The  arteries  to 
the  stomach  and  intestine 
lie  in  the  mesentery  (Fig. 
143)  and  their  course  may 
be  readily  traced  in  a  dis- 
section. 

18.  The  Course  and 
Branches  of  the  Venae  Cavae. 
—  The  blood  which  has 
thus  been  distributed  from 
the  aorta  returns  to  the 
opposite  side  of  the  heart 
through  the  veins  which 
ultimately  form  the  two 
vense  cavse.  In  general,  it 
may  be  stated  that  the 
veins  of  those  organs  which 
are  anterior  to  the  dia- 
phragm form  the  superior 
vena  cava,  while  those 
posterior  to  the  diaphragm 
form  the  inferior  vena 
cava.  The  larger  veins  usu- 
ally run  near  and  approxi- 
mately parallel  to  the  larger  arteries.  This  is  the  case  with 
those  from  the  arms  and  legs,  the  kidneys,  and  the  muscles 
of  the  trunk.  One  notable  and  very  important  exception, 


FIG.  17.  The  general  arrangement 
of  the  nervous  system  (dorsal 
view) 


STRUCTURE  OF  THE  MECHANISM  27 

however,  is  found  in  the  venous  supply  of  the  stomach, 

spleen,  and  intestines,  the  veins   of  which 

unite  to  form  a  single  large  vein  (portal 

vein)   which  passes  to  the  liver,  where 

it  breaks  up  into  smaller  vessels ;  the 

blood  which  has  thus  passed  through  the 

liver  is  finally  collected  in  the  hepatic 

vein  and  poured  by  this  into  the  inferior 

vena  cava  just  before  the  latter  passes 

through  the  diaphragm  on  its  way  to 

the  right  ventricle  (Fig.  15). 

19.  The  Capillaries.  — The  blood 
which  enters  an  organ  through  the  ar- 
teries passes  to  its  veins  through  a  sys- 
tem of  microscopic  tubes  (Fig.  16),  the 
capillaries  (Latin,  capilla,  a  hair) ;  these 
may  be  readily  seen  under  the  micro- 
scope in  the  web  of  a  frog's  foot.    From 
the  foregoing  description  of  the  course 
of  the  circulation  it  will  be  observed  that 
generally  the  blood  must  pass  through 
one  set  of  capillaries  in  going  from  the 
aorta  to  the  vense  cavse,   or  from  the 
pulmonary  artery  to  the  pulmonary  vein ; 
but  the  blood  which  flows  through  the 
capillaries  of  most  of  the  abdominal  or- 
gans (stomach,  intestines,  spleen)  must 
pass  also  through  a  second  set  of  capil- 
laries, viz.  those  of  the  liver,  before  it 
can  return  to  the  heart. 

20.  Organs  of  the  Nervous  System.  - 
The  skull  and  the  spinal  column  (p.  18) 
are  chiefly  occupied  by  the  brain  and 

the  spinal  cord  respectively,  and  from  Fm  lg    N~rye  trunks 
each  of  these  principal   organs  of  the      Of  the  right  arm 


28  THE  HUMAN  MECHANISM 

nervous  system  branches  consisting  of  cords  of  nervous 
substance,  the  nerves,  pass  out  through  small  holes  in  the 
skull  or  spinal  column  and  are  distributed  to  all  the  other 
organs,  where  they  terminate  in  peculiar  structures  called 
end  organs.  The  optic  nerve,  for  example,  ends  in  the  retina, 
the  auditory  nerve  in  the  inner  ear,  and  motor  nerves  in 
muscles,  —  the  nerve  endings  in  these  different  organs 
differing  materially  in  structure  and  arrangement. 

Figure  17  gives  some  idea  of  the  general  arrangement 
of  the  nervous  system.  The  nerves  to  the  shoulder,  arm, 
and  hand  will  be  seen  to  arise  from  the  cervical  region  of 
the  spinal  cord ;  those  for  the  trunk,  from  the  dorsal  and 
lumbar  regions ;  those  for  the  legs,  from  the  sacral  region. 
The  head  and  face  receive  nerves  from  the  posterior  por- 
tions of  the  brain.  The  dissection  of  the  arm  in  Fig.  18 
shows  more  accurately  the  main  nerve  trunks  to  that 
region.  Further  information  with  regard  to  the  structure 
of  the  nervous  system  will  be  given  in  Chapters  VII, 
XIV,  and  XV. 


CHAPTER   III 

THE  FINER  STRUCTURE  OF  TWO  TYPICAL  ORGANS, 
GLANDS  AND  MUSCLES.  THE  CONNECTIVE  TISSUES. 
THE  LYMPHATIC  SYSTEM 

In  the  previous  chapter  we  have  examined  the  general 
construction  of  the  human  machine  as  regards  its  more 
conspicuous  parts  or  organs,  and  especially  their  location, — 
whether  internal  or  external,  dorsal  or  ventral,  anterior  or 
posterior,  on  the  right  or  on  the  left,  —  their  relations  to 
certain  important  cavities,  and  their  combination  to  consti- 
tute the  mechanism  which  we  call  the  human  body.  We 
must  now  push  our  examination  further  and  investigate 
the  finer  structure  of  some  of  the  more  important  parts  of 
the  machine.  For  this  purpose  we  may  select  two  typical 
organs,  a  gland  and  a  muscle,  the  one  unfamiliar,  by  name 
at  least,  to  most  people,  the  other  well  known  in  the  form 
of  steaks,  chops,  roast  beef,  and  other  meats. 

1.  What  is  a  Gland?  —  A  gland  is  a  mass  of  tissue, 
generally  softer  than  muscle,  and  of  no  special  size  or 
shape,  though  often  rounded  or  egg-shaped.  The  gland 
most  easily  seen  is  the  milk  gland  or  udder  of  the  cow. 
This  is  a  large  mass  of  soft  tissues  devoted  to  manufac- 
turing or  secreting  milk.  In  general,  glands  are  manufac- 
turing organs  for  the  preparation  of  saliva,  gastric  juice, 
bile,  tears,  sweat,  or  other  secretions.  Some  have  tubes 
or  ducts  through  which  their  secretions  are  carried  away ; 
others  have  no  such  outlets  and  hence  are  known  as  duct- 
less glands.  Glands  vary  in  size  from  some  which  are 
microscopic  to  the  huge  liver,  which  is  the  largest  single 
organ  in  the  human  body  (see  Fig.  2).  The  pancreas  or 

29 


30 


THE  HUMAN  MECHANISM 


"  sweetbread "  is  an  excellent  gland  for  the  beginner  to 
dissect  or  study. 

2.  A  Typical  Gland.  —  If  we  have  before  us  the  whole 
or  a  part  of  any  typical  gland,  we  find  that  we  are  deal- 
ing with  a  comparatively  soft  and  sometimes  even  pulpy 
mass  held  together  by  a  loose  mesh  or  network  of  harder, 
tougher,  and  more  or  less  fibrous  materials.  A  pancreas 
or  a  liver,  if  entire,  shows  conspicuous  lobes,  and  in  the 
pancreas  these  lobes  are  plainly  subdivided  into  smaller 
lobes  or  lobules.  In  favorable  specimens  tubes  may  be  seen 
connected  with  the  gland,  some  of  which  are  blood  vessels 
supplying  blood  to  the  gland,  and  one  of  which  is  a  duct 
draining  away  from  it  the  liquid  which  the  gland  has  man- 
ufactured or  secreted.  After 
a  preliminary  examination  of 
this  sort  of  some  edible  gland, 
we  may  consider  in  greater 
detail  one  of  our  own  sali- 
vary glands,  of  which  we 
have  three  on  each  side  of  the 
head,  namely,  one  parotid, 
one  sublingual,  and  one  sub- 
maxillary  gland. 

3.  The  Structure  of  the 
Submaxillary  Gland.  —  The 
two  submaxillary  glands  lie, 
one  on  each  side  of  the  face, 
FIG.  19.  Diagram  of  submaxillary  embedded  in  the  tissues  be- 
tween the  lower  jaw  and  the 

D,  its  duct;  N,  its  nerve;  A,  its  artery;  .  2     ,  , 

v,  its  vein;  T,  tongue  upper  portion  of  the  neck. 

From    each     gland    a   duct 

passes  forward  in  the  tissues  forming  the  floor  of  the 
mouth,  into  which  it  opens  by  one  of  the  small  eminences, 
or  papillce,  under  the  tongue.  Through  this  duct  the 
gland  pours  into  the  mouth  its  secretion,  saliva. 


TYPICAL  STRUCTURE  OF  ORGANS  31 

If  the  gland  were  to  be  cut  in  two  in  any  direction  with 
a  sharp  knife,  we  should  see  at  once  that  it  is  composed  of 
separate  parts,  or  lobes,  and  that  these  lobes  are  still  further 
divided  into  smaller  por- 
tions, or  lobules.  The  lob- 
ules and  lobes  are  bound 
together  with  a  rather 
loose  connective  tissue 
which  is  continuous  with 
a  somewhat  denser  layer 
surrounding  the  gland  and 
forming  its  capsule;  the 
connective  tissue  between 
the  lobes  forms  the  pri- 
mary septa  (sing.,  septum), 
and  that  between  the  lob- 
ules the  secondary  septa. 

The  relation  of  these  T 

FIG.  20.  Diagram  of  a  cross  section  of 

structures  is  shown  m  a  gland 

Fig.  20.     With  the  aid  of  Showing  its  division  by  primary  septa  (S) 

the     micrOSCODe     We     find         into  lobes>  and  b^  secondary  septa  (*) 

.*  .  into  lobules;    also   the  origin  of  the 

that    each    lobule    IS    Still        larger  branches  of  the  duct  (D)  in  the 

further    divided   by    con-       lobes  and  lobules-  The  beginnings  of 

*  the  duct  are  shown  in  Figs.  21  and  22 

nective  tissue  into  nask- 

shaped  structures  or  alveoli  (sing.,  alveolus) ;  in  these  the 
secretion,  saliva,  is  manufactured,  and  from  them  it  is  dis- 
charged into  the  duct  of  which  the  alveoli  are  the  blind 
ends  (Fig.  21). 

The  whole  gland  may  be  compared  to  a  large  bunch  of 
grapes  ;  the  main  tubular  duct  of  the  gland  branches  (in 
the  septa  of  connective  tissue)  very  much  as  the  stem  of 
the  bunch  of  grapes  branches  ;  and  just  as  the  branches 
and  subbranches  of  the  stem  lead,  when  followed  up,  to 
the  grapes  themselves,  so  the  branches  of  the  duct  lead  to 
the  alveoli  of  the  gland.  If  now  we  pack  the  bunch  of 


32 


THE  HUMAN  MECHANISM 


grapes  in  a  small  basket  of  sawdust  or  cork  waste,  as 
Malaga  grapes  are  packed,  so  that  the  sawdust  fills  up 
loosely  the  spaces  between  the  individual  grapes  and  the 
branches  of  the  stem,  we  shall  have  something  with  which 


Duct 


Alveoli 


Secreting  Cells 
Capillary  Network 


FIG.  21.  The  origin  of  the  duct  of  a  gland  in  alveoli,  together  with  the 
connective  tissue  and  blood  vessels 

to  compare  the  arrangement  of  the  connective  tissue  in 
relation  to  the  rest  of  the  gland,  —  the  sawdust  standing 
for  the  connective  tissue  in  which  the  ducts  and  alveoli 
are  embedded,  and  the  basket  for  the  capsule. 

4.  Minute  Structure  of  Ducts  and  Alveoli.  —  The  alveoli 
are,  however,  neither  empty  shells  like  glass  flasks,  nor 
solid  masses  like  grapes,  but  rather  hollow  bags  lined  with 
a  layer  of  relatively  thickish,  closely  set  cells,  all  very 
much  alike.  Each  of  these  cells  consists  of  two  portions,  — 
a  small  central  body,  the  nucleus,  and  a  larger  surrounding 
mass,  the  cytoplasm.  And  we  may  state  at  once  that  all 


TYPICAL  STRUCTURE  OF  ORGANS 


33 


organs  of  the  body  are  composed  of  cells,  differing,  it  is 
true,  in  different  organs,  or  in  different  parts  of  the  same 


FIG.  22.  Section  of  a  portion  of  a  salivary  gland  (magnified  500  diameters). 
After  Koelliker 

The  duct  d  divides  into  the  two  branches  d'  and  d",  one  of  which  ends  in  the 
alveoli,  a,  a.  Neighboring  alveoli,  a',  a',  whose  ducts  are  not  in  the  plane 
of  the  section,  are  also  shown.  In  some  cells  the  section  does  not  include 
the  nucleus,  which  would  be  in  the  preceding  or  the  succeeding  section 

organ  (as  in  the  duct  and  the  alveolus  of  the  gland) ;  but  all 
consisting  of  two  never-failing  parts,  —  nucleus  and  cytoplasm. 
The  muscle  and  the  gland  consist  of  cells,  just  as  all 
the  branches  of  the  military  service  —  the  infantry,  the  cav- 
alry, the  artillery,  the  engineers,  etc.  —  consist  of  men.  The 
cell  is  the  anatomical  or  fundamental  unit  of  these  organs, 
as  the  soldier  is  the  fundamental  or  anatomical  unit  of  the 
army;  in  both  cases  the  anatomical  units,  differing  in 
equipment  and  training,  perform  different  kinds  of  work, 
yet  have  the  same  essential  structure ;  and  the  cells  are 
combined  into  brigades,  divisions,  or  corps,  as  tissues  and 
organs ;  they  make  of  the  body  an  army  organized  to  fight 
its  way  through  the  vicissitudes  and  against  the  obstacles 
of  life. 


34  THE  HUMAN  MECHANISM 

5.  The  Structure  of  the  Biceps  Muscle.  —  The  biceps 
muscle  is  familiar  as  the  mass  of  flesh  lying  on  the  front 
of  the  upper  arm  and  bulging  somewhat  when  the  arm  is 
bent  at  the  elbow,  especially  when  one  "  feels  his  muscle," 
or  when  a  weight  is  being  lifted  by  the  hand.  Figure  23 

shows  this  muscle  with  the 
bones  to  which  it  is  attached. 
It  consists  of  two  portions :  a 
central,  thick,  red  part,  known 
as  the  belly,  soft  when  the 
muscle  is  at  rest,  hard  when  it 
is  contracted;  and  cordlike 
strings,  or  tendons,  two  at  the 
upper  end  and  one  at  the  lower, 
by  means  of  which  the  muscle 
is  attached  to  two  bones  of  the 
shoulder  girdle  and  to  one  of 
the  forearm.  When  the 


biceps  muscle  of  the 
arm  of  the   muscle   shortens  the 

The  resting  condition  is  shown  by   points    a    and    b    are    brought 

the  solid  lines,  the  contracting  cloger  together  and  the  arm  is 

condition  by  the  dotted  lines  ° 

bent,  or  flexed,  at  the  elbow. 

This  drawing  together,  or  contraction,  is  the  special  work, 
or  function,  of  muscles  in  general. 

Every  one  has  seen  the  cross  section  of  a  muscle  in  a 
raw  beefsteak.  This  shows  that  the  muscle  as  a  whole  is 
surrounded  by  a  sheath  of  connective  tissue  which  contains 
more  or  less  fat ;  septa  pass  inwards,  dividing  the  muscle 
into  lesser  red  masses  known  as  fasciculi,  or  bundles,  and 
these  are  further  subdivided  into  secondary  fasciculi  by 
secondary  septa,  very  much  as  the  gland  is  subdivided 
into  lobules. 

A  longitudinal  section  shows  that  the  fasciculi  run  from 
tendon  to  tendon,  and  microscopic  examination  proves  that 
the  general  connective  tissues  of  the  belly  of  the  muscle 


TYPICAL   STRUCTURE  OF  ORGANS 


35 


FIG.  24.  Tendon 
(highly  magnified) 

Showing  the  fiber 
bundles  separated 


are  continuous  with  that  of  the  tendon.  The  tendon  itself 
is  a  peculiarly  strong  and  inextensible 
variety  of  connective  tissue,  consisting 
chiefly  of  parallel  fibers  which  are  spe- 
cially fitted  to  transmit  to  the  bone  the 
pull  of  the  belly  of  the  muscle. 

6.  The  Muscle  Fibers.  —  Examination 
of  the  structure  of  one  of  the  finer  fascic- 
uli in  the  belly  of  the  muscle  shows  that 
it  is  composed  of  threads,  or  fibers,  which 
at  first  sight  differ  greatly  from  the  secret- 
ing cells  of  the  gland.  These  are  the 
muscle  fibers.  They  are  1  to  1J-  inches  in 
length  and  from  ^Vo  to  ^-i^  of  an  inch 
in  thickness,  thus  being  from  250  to  2500 
times  as  long  as  wide,  and  comparable  in 
shape  to  a  long  leather  shoestring  rather  than  to  a  sausage. 
Each  fasciculus  contains  hundreds  or  even 
thousands  of  fibers.  The  fibers  always  run 
lengthwise  of  the  fasciculus,  but,  as  a  usual 
thing,  do  not  extend  its  entire  length,  as  ob- 
viously follows  from  the  fact  that  a  single 
fasciculus  of  the  biceps  is  several  inches  in 
length.  The  fibers  are  inclosed  in  a  very  thin 
transparent  membrane,  the  sarcolemma,  and  are 
bound  into  bundles,  or  fasciculi,  by  the  same 
fine  connective  tissues  seen  between  the  alveoli 
of  a  gland.  To  the  end  of  the  sarcolemma  are 
attached  fine  fibers  of  connective  tissue  which 
Showing  the  pass  into  the  tendon  (Fig.  25).  Indeed,  the  fibers 
onhe^don  of  the  tendon  are  the  collected  fibers  from  the 
fibers  to  the  sarcolemmas  of  all  the  muscle  fibers.  For  this 
ma  reason  the  part  of  the  muscle  near  the  tendon  is 
"tough  meat,"  while  that  in  the  belly  of  the  muscle  is  tender, 
owing  to  the  smaller  number  of  connective  tissue  fibers. 


end  of  a 
muscle  fiber 


36 


THE  HUMAN  MECHANISM 


7.  The  Muscle  Fiber  is  a  Cell.  —  The  muscle  fiber  at  first 
sight  does  not  seem  like  the  typical  cell  already  described, 
with  nucleus  and  cytoplasm  ;  for  when  examined  in  the 
fresh  condition  the  only  obvious  points  of  structure  seen  in 
it  are  striking  cross  striations  consisting  of  alternate  dark 
and  light  bands.  It  has  been  shown,  however,  by  ingen- 
ious and  careful  study,  that  the  cross 
striations  are  optical  appearances  pro- 
duced by  the  peculiar  shape  of  extremely 
minute  longitudinal  rods  in  the  cytoplasm 
of  the  muscle  fiber,  and  that,  immediately 
under  the  sarcolemma,  there  are  numer- 
ous characteristic  nuclei  which  are  easily 
brought  into  view  by  suitable  treatment. 
Briefly,  then,  the  muscle  fiber  is  a  cell 
tvith  many  nuclei,  in  whose  cytoplasm  are 
found  peculiar  structures,  the  myofibrils  ; 
upon  superficial  examination  these  myo- 
fibrils not  only  obscure  the  nuclei  but  give 
to  the  whole  fiber  the  appearance  of  cross 
striation. 

8.  How  far  is  the  Structure  of  Glands 
FIG.  26.  Part  of  a  and  Muscles  Typical  of  All  Organs  ?- 
muscle  fiber  Both  the  gland  and  the  muscle  are  thus 
Specially  prepared  to  composed  of  cells.  Although  differing 
considerably  in  the  two  organs,  these  cells 
possess  certain  general  and  fundamental 
features  in  common,  for  each  one  contains  a  nucleus  (or 
nuclei)  and  surrounding  cytoplasm.  Is  the  same  thing 
true  of  all  other  organs?  The  muscle  and  the  gland  are 
examples  of  organs  which  do  active  work,  but  some  other 
organs  perform  purely  passive  functions.  Such  are  the 
bones,  which  do  no  work  themselves,  but  upon  which 
the  work  of  mechanical  motion  is  done  by  the  muscles  ; 
the  tendons,  which  transmit  the  pull  of  muscles;  the 


:*W        **;; 


merolsnucM 


TYPICAL  STRUCTURE  OF  ORGANS 


37 


ligaments,  which  limit  and  sometimes  guide  the  motion 
of  bones ;  and  the  connective  tissues  which  bind  together 
other  parts  of  the  body.  None  of  these  is  a  working  organ 
in  the  sense  that  a  muscle  or  a  gland  is  a  working  organ, 
and  we  are  not  surprised  to  find  that  their  structure  departs 
from  that  of  the  muscle  and  gland  in  that,  while  nucleated 
cells  are  present  in  all  of  them,  the  great  mass  of  the  organ 
is  composed  of  life- 
less matter  between 
the  cells.  In  the 
tendon  this  con- 
sists of  very 
strong  parallel 
fibers  (Fig.  24); 
the  ligament 
shows  much  the 
same  structure ; 
a  bone  consists  C 

chiefly  of  lifeless  FlG.  27.  Longitudinal  (^4)  and  transverse  (B)  sec- 
material  contain-  tions  of  bone 

ing  large  amounts  Showing  the  branching  and  communicating  canals  — 

f         '  -i  ,          in  which  are  blood  vessels  and  nerves  —  surrounded 

by  the  lifeless  bone  substance.    In  this  are  spaces 

ter,  with  Cells  ly-       connected  with  one  another  by  very  minute  chan- 

•  i  i    j  i  H6ls.      E&CIl   OI   tllGSG    SDclCCS  COlltcilllS    £1  11V1G2T  C6ll 

mg  here  and  there      shown  in  c 
in    spaces   which 

communicate  with  one  another  by  means  of  minute  chan- 
nels. The  connective  tissues,  like  that  which  binds  the 
skin  to  the  underlying  muscles,  or  that  which  forms  the 
sheath  and  septa  of  glands  and  muscles,  consists  essen- 
tially of  lifeless  fibers  running  in  all  directions  and  thus 
ready  to  limit  the  extent  of  any  pull  tending  to  separate 
unduly  the  adjacent  organs.  To  organs  and  tissues  of  this 
kind  we  may  give  the  name  of  supporting  organs  and  tissues, 
and  they  form  almost  the  sole  exception  to  the  general 
rule  that  the  essential  part  of  a  tissue  consists  of  its  cells. 


38 


THE  HUMAN  MECHANISM 


The  latter  statement  is  true  of  all  organs  and  tissues  which 
do  work,  —  the  active  organs  of  the  body.  In  the  case  of 
the  supporting  tissues  the  cells  which  they  contain  are  the 
fundamental  units  of  the  organ,  since  they  make  the  inter- 
cellular lifeless  substance.  But  the  part  which  the  organ 
plays  in  the  work  of  the  body  as  a  whole  is  performed  by 

the  lifeless  substance  (fibers,  etc.) 
which  the  cells  have  manufactured 
and  keep  in  repair. 

9.  The  Blood  Vessels  are  Closed 
Tubes  in  Connective  Tissue.  —  The 
arrangement  of  connective  tissues 
is  fundamentally  the  same  in  the 
gland  and  the  muscle.  As  their 
name  implies,  these  tissues  serve 
the  obvious  purpose  of  binding 
the  anatomical  units  into  organs, 
but  they  also  perform  other  func- 
tions equally  important. 

We  have  seen  (Chapter  II,  Sec- 
tion 19)  that  each  organ  receives 
blood  through  one  or  more  arte- 
ries, and  that  this  blood  flows  away 
from  the  organ  through  one  or 

FIG.  28.  Three  muscle  fibers   more  veins.    If  a  colored  fluid  mass 
and  an  artery  breaking  up   which  would  afterwards  set,   e.g. 
into    capillaries    between   a  warm  solution  of  gelatin  colored 
with  carmine,  had  been  forced  into 

the  arteries  before  we  began  our  examination,  we  should 
find  that  this  mass  would  everywhere  be  confined  in  a 
system  of  closed  tubes  which  merely  lie  in  the  connect- 
ive tissue.  The  artery  entering  the  muscle  branches  into 
smaller  and  smaller  arteries  in  the  general  sheath  of  the 
organ,  or  in  its  branches,  the  septa;  from  these  finer 
arteries  an  exceedingly  rich  network  of  small  thin-walled 


TYPICAL  STRUCTURE  OF  ORGANS 


39 


tubes  is  given  off  to  the  finest  connective  tissue  which 
surrounds  the  cells  themselves ;  these  tubes  are  the  capil- 
laries. They  ultimately  unite  to  form  the  larger  veins, 
which  can  be  traced  in  the  septa  to  those  veins  which 
gross  dissection  reveals  as  leaving  the  organ  (see  Fig.  19). 
Through  these  tubes  —  arteries,  capillaries,  and  veins  — 
the  blood  flows ;  and  it  is  important  for  us  to  understand 
that  it  is  everywhere  confined  to  them  in  its  passage  through 
the  organs ;  nowhere  does  it 
come  into  direct  contact  with 
the  living  cells  (save  those 
lining  the  vessels).  Whatever 
exchange  of  matter  or  energy 
takes  place  between  the  blood 
and  the  living  cells  must  be 
through  the  walls  of  the  blood 
vessels.1  These  walls  are  rela- 
tively thick  in  the  arteries, 
usually  somewhat  thinner  in 
the  veins ;  in  the  capillaries, 
however,  they  are  very  thin, 
and  it  is  through  these  thin 
capillary  walls  that  all  inter- 
changes of  matter  take  place. 
That  the  connective  tissue  sur- 
rounding the  capillaries  bears 
an  important  relation  to  the  FlG-  29-  Superficial  and  some  deeper 

-,    , .  in  lymphatics  of  the  hand 

circulation  we  shall  now  see. 

10.  The  Lymph  Spaces  of  the  Connective  Tissue.  The 
Lymph.  —  Careful  examination  shows  that  the  fine  con- 
nective tissue  within  which  the  capillaries  are  embedded 

1  The  term  ' '  blood  vessel ' '  is  sometimes  confusing  to  the  beginner,  since 
it  suggests  a  utensil  for  holding  liquids,  like  a  cup  or  kettle  or  barrel.  In 
anatomy  the  term  "vessel"  is  applied  to  tubes,  ducts,  or  canals  through 
which  blood  or  lymph  flows. 


40 


THE  HUMAN  MECHANISM 


is  not  a  solid  or  continuous  mass,  but  rather  a  mass  or 

mesh  of  extremely  fine  fibers  or 
bundles  of  fibers,  with  here  and 
there  "connective-tissue  cells" 
which  keep  the  fibers  in  repair. 
The  connective  tissue,  there- 
fore, is  everywhere  channeled 
by  irregular  spaces  running 
between  the  fibers  and  other 
structures ;  these  spaces  com- 
municate freely  with  each  other 
and  contain  a  colorless  liquid 
known  as  lymph  ;  the  spaces  of 
the  connective  tissue  may  thus 
be  conveniently  described  as 
lymph  spaces.  They  serve  as 
communicating  channels  be- 
tween the  cells  and  the  walls 
of  the  capillaries. 

11.  Origin  of  the  Lymph.— 
The  lymph  which  the  spaces 
contain  is  derived  partly  from 
water  and  soluble  food  mate- 
rials which  have  passed  through 
the  capillary  walls  from  the 
blood,  and  partly  from  material 
produced  by  the  neighboring 

cells  (see  the  next  chapter) ;  on 
FIG.  SO.  The  two  main  lymphatic    the  other  hand  the  cellg  abgorb 
trunks  (in  white),  with  their 

openings  into  the  great  veins    from  the   lymph   substances 
near  the  heart  which  the  latter  has  received 

The  larger  of  these  trunks  — that    from  faQ  blood,  while  the  blood, 
on  the  left  side  and  known  as  the     .  ,          „  ,  t       i  -i 

thoracic  duct- returns  all  the   m  turn,  takes  trom  the  lymph 
lymph  except  that  from  the  right   substances    discharged  from 

side  of  the  head,  neck,  and  the      ,  ,,  ,       , 

right  arm  and  shoulder  region       the    Cells.     The    lymph    thus 


TYPICAL  STRUCTURE  OF  ORGANS  41 

becomes  the  means  of  communication,  the  middleman,  be- 
tween the  living  cells  of  the  organs  and  the  nourishing 
blood,  and  forms  the  immediate  environment  of  the  cells 
themselves.  In  other  words,  the  cells  of  muscles,  glands, 


L 

FIG.  31.  Diagram  of  the  relation  of  the  cells  of  an  organ  to  its  blood 

vessels,  lymphatics,  and  connective  tissue 

A,  artery;   V,  vein ;  L,  lymphatic 

and  other  organs  live  in  lymph,  just  as  the  human  body  as 
a  whole  lives  in  air,  or  a  fish  in  water. 

12.  The  Lymphatics.  —  Besides  the  veins,  which  convey 
blood  away  from  an  organ,  a  second  system  of  tubes  or 
vessels  passes  out  through  the  capsule.  These  tubes  arise 
in  the  lymph  spaces  of  the  connective  tissue  and  unite 
with  similar  tubes  from  other  regions  to  form  larger  and 
larger  trunks,  known  as  lymphatics,  which  ultimately  form 
one  or  two  great  trunks  and  open  into  the  great  veins 
near  the  heart  (see  Fig.  30).  Through  these  direct  outlets 
the  surplus  lymph  of  the  organ  flows  in  a  varying  but  for 


42  THE  HUMAN  MECHANISM 

the  most  part  continuous  stream.  This  flow  of  lymph  away 
from  an  organ  is  of  the  very  greatest  importance  in  main- 
taining the  normal  environment  of  the  cells. 

13.  Function  of  the  Lymph  Flow  from  an  Organ.  —  It  is 
clear  from  inspection  of  Fig.  31  that  there  is  a  steady  flow 
of  liquid  from  the  capillaries,  through  the  lymph  spaces  of 
the  connective  tissue,  over  the  surfaces  of  the  living  cells 
or  of  any  intervening  capillaries,  to  the  lymphatics.    The 
cell  is  thus  bathed  not  by  a  stagnant  medium  but  by  one 
which  is  in  gentle  movement ;  one  which  brings  to  all  parts 
of  its  surface  the  food  which  it  needs  and  immediately 
carries  away  from  all  parts  of  its  surface  to  the  adjacent 
capillaries  the  products  of  its  activity.    By  providing  this 
outlet  from  the  lymph  spaces  the  lymphatics  render  possible 
the  movement  of  lymph  within  the  organ  itself,  whereby 
material  is  readily  transferred  from  the  cell  to  the  capil- 
laries, and  from  the  capillaries  to  the  cell. 

14.  Distribution  of  Nerves  to  Muscles  and  Glands.  —  The 
distribution  of  nerves  resembles  that  of  the  arteries,  the 
larger  nerve  trunks  being  found  in  the  septa  and  their  fine 
ultimate  branches  being  distributed  by  way  of  the  connect- 
ive tissue  which  surrounds  the  cells,  in  whose  neighbor- 
hood or  even  within  whose  substance  they  finally  end.    As 
we  shall  see  in  subsequent  chapters,  it  is  the  function  of 
the  nerves  to  arouse  the  gland  cells  or  muscle  fibers  or 
other  cells  to  activity. 

15.  Summary.  —  Disregarding  for  the   moment  those 
peculiarities  of  arrangement,  shape,  and  structure  of  the  cells 
which  are  connected  with  the  special  work  of  each  organ, 
(e.g.  the  arrangement  of  gland  cells  to  form  a  blind  tube, 
or  of  the  connective  tissue  and  fibers  of  muscle  so  as  to 
exert  a  pull  on  a  bone),  we  may  say  that  the  typical  struc- 
ture of  an  organ  would  be  represented  in  Fig.  31.    The 
whole  is  surrounded  by  a  capsule,  receives  a  blood  supply 
through  a  system  of  closed  tubes,  and  contains  the  special 


TYPICAL  STRUCTURE  OF  ORGANS  43 

cells  upon  whose  activity  its  characteristic  work  depends. 
These  cells  are  held  together  by  a  fine  connective  tissue 
whose  numerous  and  freely  communicating  spaces  contain 
a  fluid,  the  lymph,  which  is  free  to  flow  out  through  a 
second  system  of  tubes,  the  lymphatics.  Nerves  from  the 
brain  or  spinal  cord  are  also  distributed  in  the  connective 
tissue  to  the  cells  of  the  organ. 

Before  concluding  this  description  of  the  finer  structure 
of  organs,  a  word  may  be  added  with  regard  to  the  physical 
nature  of  the  cell  substance.  In  its  literal  meaning  the 
word  "  cell "  is  a  misnomer,  since  it  suggests  a  hollow 
space  inclosed  by  solid  partitions  or  walls.  Plant  cells  do, 
in  fact,  usually  have  such  walls  around  their  cytoplasm 
(Chapter  VIII),  and  this  cytoplasm  frequently  contains 
spaces  (vacuoles)  filled  with  a  solution  of  salts,  sugar,  and 
other  dissolved  material.  But  neither  the  cell  wall  nor 
vacuoles  are  of  universal  occurrence,  each  being  rarely 
found  in  the  animal  cell,  and  often  absent*  even  in  the 
plant  cell.  Fifty  years  of  thorough  investigation  has 
reduced  the  number  of  essential  cell  constituents  to  the 
cytoplasm  and  the  nucleus.  The  ultimate  structure  of 
these  is  still  unsettled ;  but  it  would  seem  that  the  cyto- 
plasm is  a  mixture  of  highly  viscous,  liquid  materials,  often 
containing  minute  granules  in  suspension,  and  possibly 
the  nucleus  is  of  a  similar  physical  character.  In  these 
fluid  substances  occur  those  "vital"  or  ulife"  processes 
to  the  study  of  which  we  now  proceed. 


CHAPTER  IV 

THE  WORK  OF  THE  HUMAN  MECHANISM  THE  RESULT- 
ANT OF  THE  WORK  OF  ITS  DIFFERENT  ORGANS 
AND  CELLS 

From  common  experience  we  know  that  the  work  of 
the  body  as  a  whole  is  the  resultant  of  the  work  of  its 
various  organs.  We  know  that  an  act  as  simple  as  the 
taking  of  food  involves  the  cooperation  of  the  arm,  ringers, 
eyes,  teeth,  tongue,  and  other  organs.  But  we  do  not  often 
realize  that  the  work  of  each  organ  is  equally  the  resultant 
of  the  work  of  its  component  cells.  Eyes,  hands,  and 
tongue  we  c,an  see  and  even  watch  at  their  work,  but  the 
cells  of  muscles  and  glands  and  nerves  are  invisible.  If 
we  could  see  them  and  delve  into  their  recesses,  we  should 
there  discover  activities  no  less  remarkable.  As  we  cannot 
do  this  directly  and  with  the  naked  eye  we  must  do  it 
indirectly  and  make  use  of  experiments,  chemical  analyses, 
microscopic  studies,  and  all  other  available  methods.  And 
we  may  again  choose  for  detailed  consideration  two  typical 
organs,  muscle  and  gland,  the  structure  of  which  we  have 
studied  in  the  last  chapter. 

1.  Physiology  of  the  Salivary  Glands.  Working  Glands 
and  Resting  Glands.  —  The  function  of  the  salivary  glands 
is  the  secretion  or  manufacture  of  saliva  for  use  in  the 
mouth,  and  one  of  the  first  things  we  notice  about  this 
act  of  secretion  is  that  it  is  not  constant  but  intermittent. 
Most  organs  have  periods  of  activity,  or  work,  followed  by 
periods  of  inactivity,  or  rest,  and  these  glands  are  no  excep- 
tion. Physiologists  frequently  speak  of  "  working  glands  " 

44 


WOEK  OF  OKGANS  AND  CELLS  45 

and  "  resting  glands."  We  all  know  that  our  own  salivary 
glands  work  more  effectively  at  some  times  than  at  others. 
The  mouth  "  waters  "  at  the  sight  of  food ;  when  we  are 
in  the  dentist's  chair  the  flow  of  saliva  often  seems  exces- 
sive ;  and  at  other  times  our  mouths  are  "  parched "  or 
»  dry." 

2.  The  Chemical  Composition  of  Saliva.  —  The  saliva  is 
sometimes  thin  and  flows  readily,  while  at  other  times  it 
is  thick  and  viscous,  or  glairy.    This  difference  is  caused 
by  the  fact  that  the  amount  of  water  in  it  varies  under 
different  conditions.    At  all  times,  however,  it  is  a  fluid 
which  consists  of  water  containing  certain  solids  in  solu- 
tion.   The  amount  of  these  solids  varies  from  five  to  ten 
parts  in  a  thousand  of  saliva,  and  they  consist  chiefly  of 
three  groups  of  compounds.    The  first  is  mucin,  familiar 
to  us  as  the  chief  constituent  of  the  phlegm  or  mucus 
discharged  from  the  nose  and  throat,  and  giving  to  the 
fluid  its  viscous  character;  the  second  group  consists  of 
substances  known  as  enzymes,  which  have  the  power  of 
changing  starch  to  sugar;  these  we  shall  study  in  detail 
in  the  chapters  on  digestion ;  the  third  group  consists  of 
mineral  or  inorganic  salts,  of  which  ordinary*  table  salt,  or 
sodium  chloride,  is  the  most  important.    As  we  shall  see, 
the  salts  and  water  are  derived  directly  from  the  blood, 
while  the   mucin  and  enzymes  are  manufactured  by  the 
gland. 

3.  Blood  Supply  of  the  Working  Gland.  —  Whenever  a 
gland    is  actively  working  there  is  an  increased  flow  of 
blood  through  it.    For    this  reason  the    resting  gland  is 
slightly  pink,  while  the  working  gland  becomes  distinctly 
red.    Since  the  secretion  of  saliva  requires  water,  and  this 
can  be  obtained  only  from  the  blood,  it  is  easy  to  see  why 
an  abundant  blood  supply  is  essential  to  activity.    Other 
constituents    of   the  saliva,  such  as  the  inorganic  salts, 
likewise  come  directly  from  the  blood. 


46  THE  HUMAN  MECHANISM 

4.  The  Relation  of  Nerves  to  Gland  Work.  Irritability.  — 
Nerves  pass,  as  we  have  seen  (p.  30),  from  the  central  nerv- 
ous system  to  the  salivary  glands.    These  nerves  are  essen- 
tially bundles  of  nerve  fibers   which  are  distributed  to 
the  neighborhood  of  the  gland  cells,  their  final  endings 
not  yet  having  been  conclusively  demonstrated.    But  it  is 
known  that  they  are  the  means  of  conveying  to  the  gland 
an  influence,  called  a  nervous  impulse,  and  that  nervous 
impulses  cause  the  gland  to  secrete  saliva.    It  is  also  a  fact 
that  when  these  nerves  are  cut  or  injured  in  any  way,  so 
that  the  gland  is  no  longer  in  connection  with  the  brain 
and  spinal  cord,  saliva  is  not  secreted,  even  when  food  is 
placed  in  the  mouth.    Evidently  the  activity  of  the  gland 
is  normally  aroused  by  nervous  impulses  from  the  brain 
and  spinal  cord,  just  as  the  activity  of  a  receiving  instru- 
ment in  a  telegraph  office  is  aroused  by  the  electric  current 
which  comes  to  it  over  a  wire,  or  as  a  mine  is  exploded  by 
the  same  means.    The  gland  then  stands  ready  for  the  act 
of  secretion,  and  is  thrown  into  activity  by  a  nervous 
impulse  from  the  central  nervous  system.    We  speak  of 
this  action  of  a  nerve  upon  the  organ  in  which  its  fibers 
end  as  stimulation,  and  that  property  of  an  organ  in  virtue 
of  which  it  may  be  aroused  by  a  stimulus  as  irritability. 

All  the  working  organs  of  the  body  (in  contradistinction 
to  the  supporting  organs,  p.  37)  are  in  this  sense  irritable, 
and  most  of  them  receive  nerves  which  set  them  to  work. 
Irritable  tissues  may,  however,  be  artificially  stimulated 
by  other  means  than  by  nervous  impulses.  Of  these 
means  an  electric  shock  is  the  most  familiar ;  others  are 
the  sudden  application  of  heat,  the  presence  of  certain 
foreign  substances  in  the  blood,  and  even  a  sharp  blow. 

We  have  now  to  inquire  what  it  is  that  happens  in  the 
gland  when  it  is  stimulated  by  a  nervous  impulse. 

5.  The  Response    of    the  Gland   to    Stimulation  by  its 
Nerve.  —  The  visible  result  of  stimulation  of  the  gland  is 


WOEK  OF  ORGANS  AND  CELLS  47 

the  discharge  of  saliva  into  the  mouth.  Something  must 
have  happened  in  the  gland  which  has  led  to  the  passage 
of  water  and  other  substances  from  the  blood  (and  lymph) 
through  the  gland  cells  into  the  duct.  But  something  more 
has  happened,  for  the  saliva  contains  several  substances 
which  are  not  found  in  the  blood.  The  gland  has  evidently 
contributed  something  to  the  saliva.  How  were  these  con- 
tributions to  the  secretion  made  ? 

When  a  gland  has  been  resting  for  some  time  micro- 
scopic study  shows  that  the  cytoplasm  of  its  cells  becomes 
loaded  with  small  granules,  at  times  so  numerous  as  to 
obscure  the  nucleus  itself.  As  secretion  goes  on  these 
granules  disappear  from  the  cell,  presumably  contributing 
something  to  the  secretion.  If  the  secretion  continue  for 
several  hours,  it  is  found  that  the  granules  have  disap- 
peared and  that  the  size  of  the  cell  is  often  distinctly 
smaller  than  before  secretion  began. 

The  "  resting  "  gland  is  therefore  by  no  means  an  idle 
gland,  but  gradually  stores  within  its  cytoplasm  something 
in  the  form  of  granules,  which  under  the  influence  of 
nervous  impulses  or  other  forms  of  stimulation  more  or 
less  rapidly  disappear  in  the  "secretion." 

6.  Activity  of  the  Gland  involves  Chemical  Change  within 
its  Cells.  —  It  might  be  supposed  that  the  granules  manu- 
factured during  rest  are  merely  dissolved  or  washed  out 
of  the  cells  in  the  copious  stream  of  water  and  salts  which 
during  secretion  passes  through  from  the  blood  and  lymph 
to  the  duct.  If  this  were  so,  it  would  be  possible  to  dis- 
solve from  the  gland  a  substance  exhibiting  in  general  the 
same  properties  as  the  secretion  itself.  But  this  is  not  gen- 
erally the  case.  Extracts  of  fresh  glands  commonly  fail 
to  exhibit  the  characteristic  properties  of  normal  secretions, 
and  we  are  compelled  to  believe  that  the  activity  of  a  gland 
means  something  more  than  the  mere  discharge  of  previ- 
ously stored  substances ;  that  is  to  say,  the  material  of  the 


48  THE  HUMAN  MECHANISM 

granules  in  the  resting  cells  is  not  simply  set  free  when 
the  gland  secretes,  but  is  at  the  same  time  chemically 
changed.  In  the  digestive  juices,  for  example,  we  have  ac- 
tive substances  called  enzymes,  which,  it  has  been  shown, 
are  derived  from  other  substances  called  zymogens,  in  the 
gland  cells.  The  chemical  change  from  the  one  into  the 
other  is  as  essential  to  the  process  of  secretion  as  is  the  vis- 
ible flow  from  the  duct. 

These  facts  then  present  to  us  the  picture  of  the  cell  as 
the  working  or  physiological  unit,  as  we  have  already  seen 
that  it  is  the  anatomical  unit  of  the  gland  (p.  32).  The 
work  of  the  gland  is  the  sum  of  the  work  of  its  constituent 
cells.  During  the  period  of  rest  these  cells  manufacture 
from  the  blood  zymogens  or  other  substances  which  they 
store  away  in  the  form  of  granules  within  their  cytoplasm. 
When  they  are  stimulated  by  the  nervous  impulse  a  chem- 
ical change  takes  place  in  them,  the  zymogens  are  changed 
to  enzymes  and  other  substances,  and  these,  together  with 
the  water,  salts,  etc.,  derived  from  the  blood,  form  the 
secretion. 

7.  Physiology  of  Muscular  Contraction.  —  At  first  sight 
muscles  and  glands  seem  to  differ  in  action  or  function 
no  less  than  in  form  and  structure.  No  two  acts  are 
apparently  more  unlike  than  lifting  a  weight  by  the  mus- 
cles of  the  arm  and  the  secretion  of  saliva  by  the  salivary 
glands.  But  beneath  obvious  and  important  differences 
there  are  profound  and  fundamental  similarities  in  the 
processes  which  occur  in  the  two  organs  during  activity. 
Like  the  gland,  the  muscle  is  set  to  work  or  stimulated  by 
a  nervous  impulse ;  its  contraction  is  accompanied  by  an 
increased  blood  supply;  and,  most  important  of  all,  the 
work  or  contraction  is  accompanied  —  indeed  preceded  - 
by  chemical  changes  much  more  profound  than  that  of  the 
transformation  of  zymogen  into  enzyme.  These  chemical 
changes  supply  the  power  for  the  work. 


WOEK  OF  OEGANS  AND  CELLS  49 

Muscles  retain  the  power  of  contraction  for  a  time  after 
removal  from  the  body.  If  such  isolated  muscles  are  in- 
closed in  an  air-tight  space  and  made  to  contract  by  elec- 
tric stimulation,  carbon  dioxide  (CO2)  gas  will  be  given  off 
and  sarcolactic  acid  (C3H6O3)  will  be  found  in  the  muscle. 
Thus  the  activity  of  the  muscle,  like  that  of  the  gland, 
involves  chemical  change  ;  and  just  as  glandular  activity 
produces  an  output  called  a  secretion,  so  muscular  activity 
produces  an  output  consisting  of  substances  usually  de- 
scribed as  waste  products. 

8.  Combustion  or  Oxidation  in  Working  Muscle.  —  The 
source  of  the  carbon  dioxide,  sarcolactic  acid,  and  other 
waste  products,  as  of  the  mucin,  enzymes,  and  other  sub- 
stances produced  by  the  active  gland,  must  in  the  long  run 
have  been  the  matter  taken  into  the  body  of  the  animal  in 
the  food.    When,  however,  we  compare  the  intake  of  food 
with  the  output  of  waste  products  of  muscular  contraction, 
we  find  that  whereas  the  food  is  poor  in  oxygen  the  waste 
products  are  rich  in  that  element.    Somewhere,  between  the 
entrance  of  the  food  and  the  oxygen  into  the  body   and 
their  final  outgo  as  waste  products,  the  two  have  chemic- 
ally combined,  very  much  as  the  coal  and    the    oxygen 
which  enter  the  furnace  of  an  engine  combine  to  form  the 
gases   which  are  discharged  from  the  smokestack.     This 
combination    of    substances  with    oxygen   always    occurs 
when  anything  is  burned,  a  process  which  is  called  com- 
bustion or  oxidation.    The  work  of  a  muscle,  like  that  of 
the  engine,  is  the  result  of  such  combustion  or  oxidation. 

9.  Storage  of  Fuel  Substances  in  the  Muscle.  —  In  one 
respect,  however,  the  oxidation  which  gives  rise  to  mus- 
cular contraction  differs  from  that  which  gives  power  to  a 
steam  engine.    At  the  time  of  the  work  the  engine  requires  a 
draft,  i.e.  a  constant  supply  of  free  oxygen  to  its  furnace ; 
shut  this  off  and  the  fire  goes  out,  —  the  engine  comes 
to  rest. 


50  THE  HUMAN  MECHANISM 

With  the  muscle  this  is  not  so.  An  isolated  muscle, 
from  which  every  trace  of  free  1  oxygen  is  removed,  may  be 
made  to  work  in  a  vacuum,  and  in  doing  so  it  produces 
carbon  dioxide  and  other  waste  products  just  as  it  does 
when  it  is  surrounded  by  air.  The  presence  of  free  oxygen 
at  the  time  is  not  necessary  to  muscular  work  or  to  the 
production  of  waste  products  by  the  muscle. 

To  account  for  this  remarkable  fact  we  must  suppose  that 
the  muscle  has  the  power  of  storing  oxygen  within  itself, 
not  as  free  oxygen  but  in  combination  with  other  substances 
which  upon  the  application  of  a  stimulus  readily  yield  it 
up  to  combine  with  the  elements  of  the  food,  thus  forming 
the  waste  products. 

We  cannot  attempt  to  explain  in  an  elementary  work 
the  details  of  this  process ;  but  enough  has  been  said  to 
show  that  during  the  period  of  rest  the  muscle,  like  the 
gland,  has  been  engaged  in  storing  something  within  itself. 
As  the  gland  stores  its  zymogens  ready  to  undergo  chemical 
change  into  the  enzymes,  so  the  muscle  stores  from  the 
food  and  oxygen  what  we  may  term  fuel  substances.  It  is 
these  which  readily  change  into  waste  products  when  the 
muscle  is  stimulated,  thus  completing  the  analogy  so  far 
as  chemical  activity  of  the  two  organs  is  concerned. 

10.  Nature  of  the  Stored  or  Fuel  Substances  of  Muscle. 
—  The  stored  material  of  a  muscle  is  not  in  the  form  of 
granules,  as  is  that  of  the  gland,  nor  has  it  been  extracted 
from  muscles,  as  zymogens  have  been  extracted  from 
glands.  But  it  is  clear  that  the  material  made  by  the 
muscle  fiber  from  the  food  and  oxygen  must  consist  of 
unstable  bodies,  like  gunpowder  or  dynamite,  ready  to  go 
off,  as  it  were,  at  the  slightest  touch ;  and,  as  in  the  case 
of  these  explosives,  the  change  to  the  stable  substances 
produced  by  their  discharge  gives  the  power  for  work. 

1  i.  e.  oxygen  uncombined  with  other  elements,  as  the  oxygen  in  the 
atmosphere. 


WOEK  OF  OKGANS  AND  CELLS  51 

The  comparison  with  gunpowder  is  so  instructive  that  we 
may  enter  into  it  in  greater  detail.  Gunpowder  is  a  mix- 
ture of  charcoal,  sulphur,  and  saltpeter  and  contains  no  free 
oxygen.  The  oxygen  which  the  mixture  contains  is  chemic- 
ally combined  with  potassium  and  nitrogen  in  the  salt- 
peter (KNO8),  and  upon  the  application  of  an  appropriate 
stimulus,  in  the  form  of  a  spark  for  example,  this  oxygen 
is  given  up  to  unite  with  the  carbon  of  the  charcoal  accord- 
ing to  the  following  equation  : 

3C      +       S      +        2KNO3  =  3CO2       +       N2       +        K2S 
carbon        sulphur        saltpeter       carbon         nitrogen       potassium 

dioxide  sulphide 

Here  again  we  may  speak  of  the  storage  of  oxygen  during 
the  chemical  processes  by  which  the  saltpeter  was  made, 
just  as  the  muscle  has  stored  oxygen  received  from  the 
blood  in  the  form  of  some  substance  which  readily  gives 
it  up,  upon  the  application  of  a  stimulus,  —  whether  a 
nervous  impulse,  an  electric  shock,  or  a  sharp  blow,  —  to 
form  with  the  carbon  and  hydrogen  derived  from  the  food 
the  highly  oxidized  and  stable  waste  products. 

11.  Similarity  between  the  Work  of  a  Gland  and  that 
of  a  Muscle.  —  These  two  tissues  (glandular  and  muscu- 
lar) comprise  a  very  large  part  of  the  organs  of  the  body. 
Among  glands  are  the  salivary  glands,  those  in  the  lining 
of  the  stomach  and  intestine,  the  pancreas,  the  liver,  the 
thyroid,  the  tear  glands,  and  the  kidney.  The  skeletal 
muscles  comprise  nearly  half  the  total  weight  of  the  body ; 
while  among  the  muscular  or  contractile  tissues  must  be 
classed  the  heart,  half  or  more  of  the  walls  of  the  stomach, 
the  intestine,  the  arteries,  and  the  veins. 

In  the  present  chapter  we  have  described  in  broad  out- 
line the  work  of  these  two  organs  in  order  that  the 
student  may  see  for  himself  that  they  present  striking 
points  of  similarity ;  and  these  points  of  similarity  are  the 


52  THE  HUMAN  MECHANISM 

very  points  which  are  typical  of  the  action  of  most  living 
cells.     They  may  be  summed  up  as  follows: 

1.  Both  gland  cells  and  muscle  fibers  manufacture  from 
raw  materials  of  the  blood  something  which  they  store 
within  themselves  (zymogen,  fuel  substance). 

2.  When  the  gland  cell  or  muscle  fiber  which  contains 
these  substances  is  set  to  work  or  stimulated  by  a  nervous 
impulse,  or  in  other  ways,  a  chemical  change  takes  place, 
and  the  stored  substance  disappears.    At  the  same  time 
products  of  the  chemical  change  made  their  appearance, 
e.g.  the  enzymes  or  ferments  of  certain  glands  and  the 
"  waste  products  "  of  muscular  activity. 

3.  In  all  the  cells  of  the  animal  body  the  general  char- 
acter of  these  working  chemical  changes  is  that  of  a  com- 
bustion or  oxidation,  certainly  at  times  indirect,  possibly 
at  other  times  direct. 

12.  Cells  as  Factories.  —  These  and  similar  chemical 
activities  lie  at  the  basis  of  the  work  of  every  cell  of  the 
human  mechanism.    Not  only  in  the  cells  of  glands  and 
muscles,  not  only  in  all  the  other  cells  of  higher  animals, 
but  in  those  of  lower  animals  and  of  plants  as  well,  oxida- 
tion and  other  chemical  changes  are  invariable  accompani- 
ments of  life.    The  exact  nature  of  the  change  differs  in 
different  cells,  and  is  adapted  to  the  work  which  the  cell 
must  perform  for  the  body  as  a  whole.    Hence  the  universal 
need  of  carbon  and  hydrogen  and  nitrogen  in  the  food,  and 
of  oxygen  obtained  in  one  way  or  another.    The  cell  is 
fundamentally  a  chemical  laboratory,   or,   better  still,   a 
chemical  mechanism  fitted  to  employ  chemical  processes 
which  occur  within  it  for  definite  and  useful  ends. 

13.  The  Relation  of  Chemical  Change  to  the  Work  of  Mus- 
cular Contraction.  —  The  student  will  readily  recall  many 
examples  which  show  that  the  act  of  combustion  supplies 
the   power   necessary  for  doing   work  upon  surrounding 
objects.    The   explosion  of   gunpowder  will  shiver  solid 


WORK  OF  ORGANS  AND  CELLS  53 

rock ;  a  hogshead  of  water  may  be  evaporated  by  the  heat 
of  a  fire  and  its  vapor  conveyed  by  tubes  to  other  places 
where  it  may  be  condensed  again  into  water ;  here  the  act 
of  combustion  has  just  as  truly  done  work  by  moving  the 
hogshead  of  water  as  if  the  water  had  been  moved  from 
the  one  place  to  the  other  by  human  effort.  In  both  these 
cases  it  is  comparatively  easy  to  trace  every  step  of  the 
process  and  thus  see  clearly  the  relation  between  the  com- 
bustion and  the  work  done.  In  the  case  of  the  muscle 
fiber,  on  the  other  hand,  the  relation  is  not  so  obvious,  but 
only  because  we  do  not  so  readily  see  the  plan  and  mech- 
anism of  its  constituent  parts. 

A  man  who  has  lived  all  his  life  as  a  hunter  in  the  woods 
and  sees  for  the  first  time  a  shoe  factory  may  not  under- 
stand how  the  combustion  of  coal  in  the  furnace  of  the 
engine  does  so  many  and  varied  things,  because  he  is  not 
familiar  with  the  construction  and  operation  of  machinery. 
We  ourselves  are  in  a  somewhat  similar  position  with 
regard  to  the  muscle  fiber  ;  but,  like  the  factory,  it  is  a 
mechanism  so  arranged  that  the  power  supplied  by  oxida- 
tion of  food  is  applied  to  accomplish  a  certain  end,  —  the 
shortening  or  contraction  of  the  fiber  in  which  it  occurs, 
and  so  the  lifting  of  a  weight. 

It  is  now  generally  believed  that  the  rodlike  myofibrils 
(p.  36)  form  an  essential  part  of  this  mechanism.  But  we 
cannot  discuss  this  question  more  extensively  at  the  present 
time.  Our  only  purpose  in  introducing  it  is  to  give  to  the 
student  that  most  fundamental  conception  of  physiology, 
—  cells  as  living  mechanisms  capable  of  using  the  power 
obtained  from  the  oxidation  of  the  food  to  do  definite 
kinds  of  work. 

14.  Recapitulation.  —  The  present  chapter  is  entitled 
The  Work  of  the  Human  Mechanism  the  Resultant  of  the 
Work  of  its  Different  Cells.  We  have  traced  the  character 
of  the  work  done  in  the  case  of  the  gland  and  the  muscle, 


54  THE  HUMAN  MECHANISM 

and  have  found  that  it  is  fundamentally  the  work  of  the 
cells  of  which  the  organs  are  composed.  The  cells  of  other 
organs  are  similarly  constructed  to  do  other  kinds  of  work, 
and  the  character  of  their  chemical  changes  and  of  the 
mechanisms  for  utilizing  power  varies  accordingly;  all, 
however,  showing  the  same  fundamental  plan  of  working 
engines.  We  can  now  readily  appreciate  the  meaning  of 
the  heading  of  the  chapter  ;  the  body  is  a  community  of 
groups  of  cells  of  different  kinds,  each  kind  doing  some 
work  more  or  less  peculiar  to  itself  ;  in  addition  to  the  two 
groups  (gland  and  muscle  cells)  which  we  have  studied, 
there  are  nerve  cells  in  the  brain,  spinal  cord,  and  else- 
where ;  cells  which  make  blood  corpuscles  ;  cells  which 
keep  in  repair  the  connective  tissues,  —  bone,  gristle,  ten- 
don, and  ligament;  and  many  more,  such  as  cells  which 
manufacture  or  themselves  form  the  lining  of  free  sur- 
faces, like  the  skin,  the  alimentary  tract,  the  air  passages, 
etc.  The  sum  total  or  net  result  of  the  activities  of  these 
and  other  cells  makes  up  the  work  of  the  body  as  a  whole ; 
the  work  of  the  body  —  the  human  organism,  the  human 
mechanism  —  is  thus  the  outcome  or  resultant  of  the  work 
of  its  different  component  cells. 


CHAPTER  V 
WORK,  FATIGUE,  AND  RESTORATION 

While  it  is  true,  as  shown  in  the  last  chapter,  that 
capacity  for  work  is  one  of  the  principal  characteristics  of 
the  human  body,  no  experience  of  daily  life  is  more  familiar 
than  that  work  is  followed  by  fatigue.  This  is  true  both 
of  individual  organs  and  of  the  organism  as  a  whole ;  for 
fatigue  may  be  either  local,  as  when  some  one  muscle  is 
tired  from  hard  work,  or  general,  as  when  weariness  affects 
all  organs,  —  those  which  have  been  resting  as  well  as  those 
which  have  been  working. 

We  use  the  word  "  fatigue  "  in  two  different  senses,  and 
it  is  important  that  a  distinction  be  clearly  made  between 
them.  In  the  one  sense  the  word  means  the  diminution  of 
working  capacity  due  to  work.  In  testing  one's  strength  of 
grip  or  of  back  a  second  test  if  made  immediately  shows 
less  work  done  than  at  the  first  test,  and  this  is  true 
whether  or  not  we  are  conscious  of  fatigue  or  of  diminished 
working  power.  If,  however,  a  certain  time  be  allowed 
for  rest,  the  second  test  will  give  as  good  results  as  the 
first. 

In  the  other  sense  the  word  refers  to  the  feeling  of  fatigue 
which  frequently,  though  not  always,  accompanies  the 
diminution  of  working  power.  We  may  even  "  feel  tired" 
when  we  have  been  doing  nothing,  and  conversely,  under 
the  influence  of  excitement  or  other  causes,  we  may  experi- 
ence no  feeling  of  fatigue  even  when  we  are  near  the  limit 
of  our  working  power.  Often  in  an  exciting  game  the 
players  do  not  know  at  the  time  that  they  are  tired  or  even 

55 


56 


THE  HUMAN  MECHANISM 


that  their  working  power  is  lessened ;  and  stories  are  told 
of  soldiers  in  hasty  retreat  who  feel  that  they  must "  drop  in 
their  tracks  "  until  the  discharge  of  musketry  close  behind 
stimulates  them  to  move  faster  than  ever.  The  report  of 
the  rifles  did  not  change  the  working  power  of  the  mus- 
cles, but  only  removed  for  the  time  being  the  feeling  of 
weariness  in  the  nervous  system,  which  had  interfered 
with  getting  from  these  muscles  their  best  work. 

The  consciousness  of  fatigue  has  its  seat  in  the  nervous 
system,  and  its  study  must  be  postponed  until  we  have 
learned  something  of  the  physiology  of  the  brain  and 
spinal  cord.  In  the  present  chapter  we  are  not  immedi- 
ately concerned  with  this  side  of  the  question,  but  rather 
with  the  diminution  of  working  power  produced  by  work. 
Such  fatigue  must  be  measured  not  by  our  sensations  but 
by  the  work  accomplished,  whether  that  work  be  physical 

or  mental.  And  as 
we  studied  the  physi- 
ology of  work  in  its 
simplest  form  in  a 
single  working  or- 
gan, such  as  a  muscle 
or  a  gland,  so  we  can 
best  begin  our  study 
of  diminished  work- 
ing power  or  fatigue 
in  one  of  these  same 
organs,  namely,  the 
skeletal  muscle. 

1.  Fatigue  of  an 
Isolated  Muscle  and 
of  a  Muscle  with 

Intact  Circulation.  —  The  course  of  fatigue  in  a  muscle 
is  best  studied  by  causing  the  muscle  to  contract  to  its 
utmost,  at  regular  intervals  of  time,  against  the  resistance 


FIG.  32.  Diagram  of  apparatus  for  recording 
successive  muscular  contractions 


FATIGUE  AND  KESTOBATION 


57 


of  a  suitable  spring.  If  now  we  record  the  height  of  each 
contraction,  we  obtain  a  se- 
ries which  shows  at  once 
the  effect  of  the  work  on 
the  working  power,  i.e.  the 
course  of  fatigue.  Fig.  32 
gives  a  diagram  of  the  ar- 
rangement of  such  an  ex- 
periment with  an  isolated 
muscle,  i.e.  a  living  muscle 
detached  from  the  rest  of 
the  body.  One  tendon  is 
made  fast  in  a  rigid  clamp, 
while  the  other  is  attached 
to  the  spring,  which  is 
stretched  by  the  contrac- 
tion when  the  muscle  is 
stimulated.  The  length  of 
the  line  written  by  the 
lever  AB  records  what  the 
muscle  is  capable  of  doing 
at  the  time;  and  if  the 
records  of  successive  con- 
tractions are  made  on  the 
smoked  surface  of  a  slowly 
revolving  drum,  as  in  the 
figure,  we  have  at  once  a 
record  of  the  course  of 
fatigue. 

Such  fatigue  tracings 
may  also  be  taken  from  a 
muscle  within  the  body, 
and  hence  with  its  circu- 
lation intact.  Thus  the 


FIG.  33.  Kecord  of  the  successive  con- 
tractions of  the  flexor  muscles  of 
the  elbow  joint 


work  of  the  biceps  muscle 


Showing  the  gradual  decrease  in  working 
power  to  a  fatigue  level.  The  muscle 
contracted  once  every  three  seconds 
against  the  resistance  of  a  strong 
spring,  which  was  stretched  each  time 
as  far  as' the  strength  of  the  muscle 
permitted 


58  THE  HUMAN  MECHANISM 

in  flexing  the  arm  at  the  elbow  (Fig.  23)  may  be  recorded 
by  instruments  essentially  similar  to  that  used  with  the 
excised  muscle.  And  in  Fig.  33  we  have  reproduced  a 
tracing  of  this  kind. 

It  is  evident  that  a  continuous  line  joining  the  highest 
points  reached  by  the  several  contractions  will  represent 
graphically  the  course  of  fatigue,  and  in  Fig.  34  the  line  a 
represents  this  so-called  "  curve  of  fatigue  "  in  the  experi- 
ment whose  results  are  given  in  Fig.  33.  It  falls  off  at 
first  rather  rapidly,  then  more  and  more  slowly,  until  at 
last  it  becomes  parallel  with  the  base  line.  In  other  words, 
the  muscle  in  this  case  finally  finds  a  constant  level  of 
working  power.  This  may  be  called  the  fatigue  level. 

The  broken  line  b  in  Fig.  34  gives  the  result  of  a 
fatigue  tracing  with  the  isolated  muscle.  It  will  be  seen 


FIG.  34.  Curves  of  fatigue 
a,  from  a  muscle  with  intact  circulation;  6,  from  an  isolated  muscle 

that  the  fall  in  the  height  of  contraction  continues  until  at 
last  the  muscle  no  longer  responds  to  stimulation.  The 
contrast  thus  brought  out  between  the  effect  of  work  upon 
muscles  with  and  those  without  the  circulation  shows  that 
the  circulation  of  the  blood  through  the  working  organ  in 
some  way  maintains  the  working  power. 

The  height  of  the  fatigue  level  in  the  same  muscle  at 
different  times  is  very  closely  dependent  on  the  rate  at 


FATIGUE  AND  RESTORATION  59 

which  the  muscle  works.  Thus  with  a  contraction  every 
four  seconds  instead  of  every  three  seconds  the  fatigue 
level  would  be  higher  than  in  Fig.  33  ;  with  a  contraction 
every  second  it  would  be  much  lower.  When  the  contrac- 
tions come  every  nine  or  ten  seconds  there  is  usually  no 
falling  off  in  the  work  done,  the  time  between  contractions 
being  sufficient  for  the  complete  recovery  of  working  power. 

This  picture  of  fatigue  hardly  agrees  with  our  feeling 
of  fatigue,  for  the  decline  of  working  power  begins  at 
once,  or  at  most  after  a  very  small  number  of  contractions, 
whereas  we  usually  notice  fatigue  only  after  work  has 
gone  on  for  a  considerably  longer  time.  One  does  not  feel 
tired  from  walking,  for  example,  during  the  first  ten  or 
twenty  minutes  of  the  walk.  We  need  not  discuss  here 
just  what  makes  us  unconscious  of  the  beginnings  of 
fatigue;  but  it  is  important  to  understand  that  whether 
we  are  or  are  not  aware  of  its  presence,  fatigue  is  the 
invariable  and  immediate  result  of  all  muscular  work. 

Weariness  is  simply  the  conscious  sensation  of  fatigue, 
but  fatigue  is  a  physical  condition  of  living  cells  and 
organs.  Moreover,  its  phenomena  are  by  no  means  con- 
fined to  muscular  work.  When  a  gland  is  stimulated  to 
vigorous  secretion  a  diminution  is  sooner  or  later  noted  in 
the  amount  of  the  secretion,  and  there  is  good  reason  to 
believe  that  nerve  cells  may  also  become  tired  from  con- 
tinued activity.  Fatigue,  then,  in  one  word,  is  a  natural 
condition  of  an  organ  accompanying  work,  and  we  may 
proceed  to  inquire  into  its  exact  cause. 

2.  Waste  Products  as  a  Cause  of  Fatigue.  —  When  blood 
which  has  been  circulating  through  a  fatigued  muscle  is 
sent  through  a  resting  muscle,  the  resting  muscle  shows 
signs  of  fatigue,  even  though  it  has  itself  done  no  work. 
Apparently  the  blood  has  extracted  from  the  working 
muscle  something  which  has  the  power  of  lessening  the 
working  capacity  of  a  fresh  muscle. 


60  THE  HUMAN  MECHANISM 

The  same  thing  is  illustrated  by  another  experiment. 
A  muscle  which  is  deprived  of  its  circulation,  e.g.  by 
clamping  its  arteries  and  veins,  is  fatigued  by  vigorous 
work ;  it  is  then  found  that  although  when  left  to  itself 
a  slight  recovery  takes  place,  this  recovery  is  much  more 
marked  if  we  first  pass  through  its  blood  vessels  a  weak 
solution  of  salt.  Here  no  food  is  supplied ;  the  salt 
solution  has  only  removed  something  from  the  fatigued 
muscle,  which  in  consequence  of  this  treatment  recovers 
some  of  its  working  power. 

Again,  the  mere  exposure  of  a  resting  muscle  to  blood 
containing  sarcolactic  acid,  or  to  blood  heavily  charged 
with  carbon  dioxide  (CO2),  produces  the  condition  of 
fatigue.  Now  in  the  last  chapter  it  has  been  shown  that 
both  sarcolactic  acid  and  carbon  dioxide  are  waste  prod- 
ucts of  muscular  activity;  and  these  and  other  facts 
have  led  to  the  view,  now  generally  received,  that  the  waste 
products  of  the  active  organ  interfere  with  the  work 
of  the  organ  and  so  constitute  one  of  the  main  causes  of 
fatigue.  It  is  apparently  for  this  reason  that  the  injection  of 
an  extract  of  worked  muscle  fatigues  fresh  muscle,  for  the 
extract  contains  waste  products.  It  is  for  the  same  reason 
that  washing  out  a  fatigued  muscle  with  salt  solution  pro- 
duces partial  recovery,  for  the  waste  products  of  activity 
are  in  this  way  partially  removed.  We  can  also  understand 
why  fatigue  always  accompanies  vigorous  work.  Waste 
products  then  necessarily  accumulate  and  clog  the  living 
mechanism,  because  they  cannot  be  removed  by  the  blood 
as  fast  as  they  are  formed  by  the  muscle  cells.  No  fatigue 
occurs  with  only  a  single  contraction  every  ten  seconds  or 
more,  because  between  contractions  sufficient  time  is  given 
to  insure  the  complete  removal  of  wastes. 

3.  Loss  of  Fuel  in  the  Working  Muscle  as  a  Cause  of 
Fatigue.  —  The  blood,  however,  not  only  removes  the 
wastes  but  also  brings  new  food  and  oxygen  with  which 


FATIGUE  AND  RESTORATION  61 

the  muscle  makes  good  the  loss  of  explosive  fuel ;  and  it 
may  well  be  —  although  it  is  not  absolutely  proved  —  that 
recovery  from  fatigue  depends  upon  both  of  these  good 
offices  of  the  blood.  We  have  certainly  one  well-estab- 
lished cause  of  fatigue,  namely,  the  presence  of  the  waste 
products  of  activity;  and  we  recognize  the  probability 
that  the  depletion  of  explosive  fuel  may  also  contribute  to 
the  result.  But  whether  the  first  of  these  causes  alone  is 
sufficient  to  explain  it,  or  whether  both  work  together,  we 
can  understand  that  the  maintenance  of  a  good  blood 
supply  is  of  the  first  necessity,  and  that  undue  fatigue 
can  be  avoided  only  by  working  at  a  moderate  rate.  It  is 
an  old  and  physiologically  true  saying  that  "  it  is  the  pace 
that  kills." 

4.  Explanation  of  the  Fatigue  Level In  the  experi- 
ment with  the  isolated  muscle  no  waste  products  were 
removed,  nor  were  new  food  and  oxygen  supplied ;  hence 
the  wastes  in  the  muscle  increased  with  each  contraction, 
until  at  last  their  accumulation  prevented  all  contraction. 
In  the  normal  muscle  the  wastes  likewise  accumulate  for 
a  time ;  and  this  is  why  the  curve  of  work  at  first  falls 
(Fig.  33).  It  does  not  continue  to  fall,  because  as  the 
wastes  within  the  muscle  increase  in  amount  the  blood 
carries  more  and  more  of  them  away  in  a  given  time.  The 
quantity  of  waste  removed  thus  continues  to  increase  until 
the  same  quantity  is  carried  away  from  the  muscle  between 
two  contractions  as  the  muscle  produces  with  each  con- 
traction. When  this  happens  no  further  accumulation  of 
waste  is  possible  and  the  fatigue  level  is  established. 

5.  General  Fatigue  resulting  from  Muscular  Activity. 
-  Every  one  knows  that  after  a  day's  tramp  it  is  not  simply 
the  worked  muscles  which  are  unfit  for  good  work,  but 
that  the  brain,  too,  is  tired,  for  hard  mental  work  is  then 
difficult  or  well-nigh  impossible;  and  it  is  generally  the 
fact  that  long-continued  muscular  work  fatigues  the  brain 


62  THE  HUMAN  MECHANISM 

more  than  brain  (mental)  work  itself.  The  obvious  explana- 
tion of  this  fact  is  that  the  waste  products  of  muscular 
activity  have  accumulated  in  the  blood  more  rapidly  than 
the  body  can  get  rid  of  them,  and  so  have  fatigued  the 
other  tissues,  including  the  nerve  cells  of  the  brain,  just 
as  the  injection  of  the  extract  of  a  tired  muscle  lessens 
the  working  power  of  a  fresh  muscle.  No  doubt  these 
same  waste  products  may  similarly  fatigue  gland  cells ; 
for  experience  seems  to  show  that  the  secretion  of  digest- 
ive juices  is  not  so  active  when  one  is  suffering  from 
muscular  fatigue,  and  that  it  is  not  wise  to  eat  heavy 
meals  when  one  is  tired  out.  We  can  also  understand  why 
long-continued,  vigorous  muscular  action  produces  marked 
fatigue  in  nerve  cells  and  gland  cells,  while  the  activity 
of  the  latter  produces  only  inappreciable  fatigue  in  the 
muscles ;  for  the  amount  of  chemical  change  and  the  pro- 
duction of  wastes  is  far  greater  in  the  case  of  muscular  work 
than  in  that  of  nervous  or  glandular  activity. 

6.  The  Analogy  of  the  Engine.  —  In  previous  chapters 
we  have  compared  the  living  body  with  a  machine  or  loco- 
motive engine ;  both  do  work,  and  both  obtain  the  power 
for  work  from  the  direct  or  indirect  oxidation  of  food  or 
fuel.  What  we  have  now  learned  about  fatigue  suggests 
an  extension  of  the  same  comparison.  Every  locomotive 
suffers  impairment  of  its  working  power  with  use,  and 
special  measures  are  taken  to  limit  this  impairment  as 
much  as  possible;  the  gases  and  smoke  are  carried  away 
at  once  by  the  chimney  or  smokestack  ;  the  furnace  is 
provided  with  a  grate  so  that  the  ashes  shall  not  accumu- 
late and  shut  off  the  draft;  the  bearings  are  oiled  and 
foreign  matters  removed;  above  all,  as  the  consumption  of 
fuel  goes  on,  the  loss  is  made  good  by  stoking. 

The  continuance  of  the  work  of  the  engine  requires  two 
things,  —  fresh  supplies  of  fuel  and  the  removal  of  wastes. 
Obviously  the  blood  performs  these  same  offices  for  the 


FATIGUE  AND  EESTOEATION  63 

cell.  It  supplies  to  the  cell  fuel  (food)  from  the  alimen- 
tary canal  and  oxygen  from  the  lungs,  and  it  carries  away 
the  waste.  Provision  is  thus  made  to  maintain  the  human 
machine  in  working  order  and  good  condition  during  its 
activity.  If  the  blood  flows  too  slowly  through  the  muscle, 
the  same  thing  happens  as  in  the  locomotive  when  the  fire- 
man neglects  to  rake  the  fire  or  to  put  on  new  fuel ;  the 
efficiency  both  of  the  human  engine  and  of  the  locomotive 
may  be  impaired  either  by  the  undue  accumulation  of  the 
waste  products  of  its  own  activity  or  by  the  neglect  to 
supply  proper  food  or  fuel. 

7.  The  Cell  a  Living  Machine.  —  The  similarity  of  the 
cell  to  the  lifeless  engine  is  more  than  a  mere  analogy. 
The  cell  is  not  only  like  —  it  actually  is  —  an  engine, 
machine,  or  mechanism,  but  it  is  also  a  living  mechanism. 
This  means  that  it  possesses  properties  and  powers  not 
shown  by  the  locomotive.  Both  may  be  injured  by  over- 
work or  by  accident,  but  only  the  living  mechanism  is 
itself  capable  of  repairing  damage;  the  locomotive  must 
be  sent  to  the  shops  and  be  repaired  by  work  done  upon 
it  by  other  machines ;  if  the  boiler  rusts,  it  must  be  taken 
out  and  a  new  one  put  in  ;  if  the  wheels  wear  unevenly, 
they  must  be  made  true  again  by  turning  in  a  lathe,  or 
new  ones  must  be  substituted ;  when  the  grate  burns  out 
a  new  one  must  be  put  in  its  place.  The  living  cell,  on 
the  other  hand,  itself  makes  these  repairs  from  the  same 
material  that  supplies  its  fuel.  Indeed,  the  material  for 
the  repair  of  the  living  cell  is  to  some  extent  the  same 
thing  as  the  fuel;  and  when  food  is  not  supplied  with 
sufficient  frequency  this  living  machine  actually  consumes 
its  own  substance  for  fuel,  —  as  happens,  for  example,  dur- 
ing periods  of  starvation.  Nothing  of  this  kind,  of  course, 
is  seen  in  any  locomotive. 

These  are  merely  suggestions  of  the  striking  differences 
between  the  living  engine  and  the  locomotive.  But  we 


64  THE  HUMAN  MECHANISM 

cannot  pursue  this  subject  further  in  an  elementary  work 
like  the  present;  enough  has  been  said  to  indicate  the 
difference,  while  the  facts  given  in  this  and  the  preceding 
chapter  show  that  so  far  as  the  resemblances  go,  they  are 
complete.  Both  machines  do  work,  both  require  power  for 
their  work,  both  derive  their  power  from  the  oxidation 
of  fuel,  both  are  provided  with  special  mechanisms  which 
transform  energy  into  definite  kinds  of  work,  both  are 
clogged  by  the  accumulation  of  waste  products,  and  hence 
both  require  care  for  their  best  efficiency.  Yet  the  full 
recognition  of  the  fact  that  the  cell  is  an  engine,  a 
machine,  a  mechanism,  should  not  make  us  forget  that  other 
fact  that  it  is  more  than  this :  that  it  is  a  living  engine,  a 
living  machine,  a  living  mechanism,  capable  not  only  of 
fatigue  but  also  of  self-repair ;  and,  perhaps  most  wonderful 
of  all,  the  organism  which  these  cells  compose  is  capable 
of  sensations  of  weariness  in  fatigue,  and  of  refreshment 
and  new  power  after  feeding  and  rest,  —  powers  and  sen- 
sations utterly  lacking  in  any  lifeless  machine  however 
perfect. 


CHAPTER  VI 
THE  INTERDEPENDENCE  OF  ORGANS  AND  OF  CELLS 

1.  Are  Cellular  "  Waste  Products "  necessarily  Harmful? 
-  We  have  now  learned  that  the  active  living  cells  of  the 
body  are  the  seat  of  chemical  changes  which  produce  new 
substances ;  that  the  accumulation  of  these  products  of 
activity  tends  to  limit  the  working  power  of  the  cells  in 
which  they  are  produced,  and  may  even  depress  the  activity 
of  other  cells  to  which  they  are  carried  by  the  blood.  In 
the  case  of  the  skeletal  muscles  we  have  spoken  of  the  car- 
bon dioxide,  the  sarcolactic  acid,  etc.,  as  "waste  products," 
meaning  thereby  that  they  are  incapable  of  serving  as 
sources  of  power  for  the  work  of  the  muscle;  and  this 
term,  together  with  the  fact  that  they  constitute  one  cause 
of  fatigue,  is  apt  to  mislead  us  into  supposing  that  they 
can  be  of  no  further  use  to  the  body,  or,  even  more,  that 
they  are  necessarily  harmful  and  that  their  presence  in  the 
blood  is  objectionable. 

These  conclusions,  however,  do  not  necessarily  follow 
from  the  facts.  It  does  not  even  follow  that  a  substance 
which  produces  fatigue  is  for  that  reason  undesirable. 
Most  adults  can  recall  times  when  because  of  long-con- 
tinued application  to  mental  work,  or  because  of  worry  or 
other  nervous  strain,  they  have  become  overexcitable  and 
restless,  and  have  been  unable  to  obtain  the  sleep  of  which 
the  body  as  a  whole  stands  in  need.  At  such  times  sleep  is 
often  best  secured  by  producing  general  fatigue  through 
muscular  work.  The  waste  products,  by  their  very  act  of 
fatiguing  the  overexcited  nerve  cells,  may  be  of  service  to 

65 


66  THE  HUMAN  MECHANISM 

the  body  as  a  whole.  And  it  is  probably  true  that  not  only 
in  such  abnormal  conditions  but  also  in  the  daily  conduct 
of  life  the  fatigue  of  moderate  muscular  activity  contrib- 
utes its  share  toward  inducing  healthful  and  refreshing 
slumber. 

Thus  far  we  have  considered  the  chemical  activities  of 
each  organ  as  contributing  to  the  work  of  the  organ  in 
which  they  occur,  and,  because  of  the  accumulation  of  waste 
products,  as  the  occasional  cause  of  undue  interference  with 
efficient  activity,  both  in  the  working  organ  and  elsewhere. 
And  yet  the  familiar  case  which  we  have  just  cited  sug- 
gests another  view  of  the  matter.  The  products  of  the 
chemical  activity  of  one  organ  may  be  of  service  to  other 
organs,  and  so  to  the  body  as  a  whole  ;  and  while  their 
too  rapid  accumulation  in  the  blood  is  undesirable,  their 
presence  in  moderate  amounts  may  be  beneficial,  and 
may  contribute  to  the  normal  environment  of  the  cells  of 
the  body. 

It  must  never  be  forgotten  that  the  secretions  of  the  sali- 
vary glands,  the  stomach,  and  the  pancreas  as  truly  contain 
products  of  cellular  activity  as  does  the  blood  which  leaves 
a  working  muscle.  It  is  also  probable,  although  it  has  not 
yet  been  directly  proved  by  experiment,  that  if  such  a  secre- 
tion were  to  accumulate  within  and  around  the  gland  cells, 
it  would  limit  their  activity  and  produce  a  true  fatigue. 
The  constituents  of  the  saliva,  for  example,  are  waste  prod- 
ucts of  the  gland,  in  the  sense  that  they  do  not  serve  for 
the  manufacture  of  new  saliva,  but  we  do  not  call  them 
waste  products  chiefly  because  we  know  they  are  still  use- 
ful to  the  body. 

2.  The  Thyroid  Gland.  —  This  view  of  the  case  is  strik- 
ingly emphasized  in. the  physiology  of  the  thyroid  gland,  — 
a  small  organ  in  the  neck,  the  two  chief  lobes  of  which 
lie  alongside  the  trachea.     For  a  long  time  its  use  was 
not  understood,  and  at  times  it  was  even  supposed  that 


INTERDEPENDENCE  OF  ORGANS 


67 


it  played  an  unimportant  part  in  the  life  of  the  body  as 
a  whole.  It  has  been  found,  however,  by  experiment 
that  removal  of  the  thyroid  is  followed  by  a  disease  in 
all  respects  similar  to  one 
which  had  long  been  ob- 
served in  human  beings,  and 
especially  in  children  ;  and 
this  fact  suggested  that  the 
disease  is  due  to  the  failure 
of  the  thyroid  to  perform 
its  normal  functions. 

The  subject  w~as  further 
cleared  up  by  the  discovery 
that  after  the  removal  of 
the  thyroid  in  a  lower  ani- 
mal the  disease  in  question 
could  be  prevented  by  feed- 
ing the  animal  thyroids  or 
even  by  giving  to  it  a  cer- 
tain substance  extracted 
from  them.  Evidently  the 
thyroid  manufactures  and 
discharges  into  the  blood  a 

peculiar  substance  necessary  to  the  healthy  life  of  the  cells 
of  the  body ;  and  when  the  gland  fails  to  manufacture  this 
substance  it  can  still  be  supplied  artificially  by  introducing 
it  into  the  blood  by  absorption  from  the  alimentary  canal. 

3.  Internal  Secretions.  —  In  our  study  of  secretion  in 
Chapter  IV  (p.  44)  we  dealt  only  with  glands  which  dis- 
charge their  principal  products  through  a  duct  into  some 
part  of  the  alimentary  canal ;  such  glands  are  the  salivary 
glands,  the  pancreas,  and  the  liver.  Other  glands  send  ducts 
to  the  surface  of  the  body,  —  for  example,  the  sweat  glands 
which  discharge  perspiration  upon  the  skin,  and  the  lachry- 
mal glands  which  discharge  the  tears  on  the  eyeball.  In  the 


FIG.  35.  Cross  section  of  the  thyroio. 
gland 

The  cells  secrete  into  the  closed  sacs, 
which  they  surround,  the  internal 
secretion,  which  then  passes  out  be- 
tween the  cells  into  the  lymph  spaces 
of  the  connective  tissue 


68  THE  HUMAN  MECHANISM 

case  of  the  thyroid,  on  the  other  hand,  we  have  an  example 
of  an  organ  which,  like  those  just  mentioned,  manufactures 
a  special  substance  from  the  blood,  but,  having  no  duct, 
contributes  the  products  of  its  manufacture  to  the  blood,  for 
the  use  of  other  cells.  This  process  is  spoken  of  as  internal 
secretion,  to  distinguish  it  from  ordinary  secretion,  in  which 
case  something  is  discharged  on  a  free  surface  like  the  skin, 
or  into  the  alimentary  canal,  the  nasal  cavity,  or  the  air 
passages. 

4.  Other  Examples  of  Internal  Secretion.  —  Certain  other 
organs  similarly  manufacture  and  contribute  to  the  blood 
material  for  use  elsewhere.  Lying  immediately  above  the 
kidney  are  two  small  bodies,  the  adrenal  or  suprarenal 
bodies,  which,  like  the  thyroids,  were  until  recently  supposed 
to  be  of^  minor  importance.  It  is  now  known  that  these 
also  contribute  to  the  blood  an  internal  secretion  which  is 
absolutely  indispensable  for  the  healthy  life  of  other  organs. 

A  still  more  remarkable  discovery  has  shown  that  the 
pancreas  not  only  manufactures  an  important  digestive 
juice  (pancreatic  juice)  which  it  discharges  into  the  intes- 
tine through  its  duct  (pancreatic  duct,  see  Fig.  50),  but 
also  produces  another  substance  which  is  necessary  in 
order  that  other  organs  may  use  the  sugar  which  is  in 
their  food.  Here  we  have  an  example  of  an  organ  which 
produces  both  an  ordinary  and  an  internal  secretion,  and 
the  same  thing  seems  to  be  true  of  the  kidney,  as  it  cer- 
tainly is  of  the  liver. 

Thus,  through  the  medium  of  the  blood  the  chemical 
activity  of  one  organ  may  affect  the  life  of  other  organs 
favorably  or  unfavorably.  All  the  cells  of  the  body  help 
to  make  the  blood  what  it  is,  many  of  them  contributing 
to  it  something  useful  or  even  necessary  to  other  cells. 
The  work  of  the  body  is  not  merely  the  sum  total  of  the 
work  of  its  separate  cells,  each  working  for  itself  alone 
and  performing  a  single  function.  Between  the  cells  an 


INTERDEPENDENCE  OF  OKGANS  69 

exchange  of  products  often  takes  place,  so  that  cells  become 
both  serviceable  to  and  dependent  upon  one  another  for  the 
material  needed  to  carry  out  their  own  special  chemical 
activities.  And  what  is  true  of  cells  is  no  less  true  of 
organs;  these  also  are  interdependent,  ministering  to  one 
another. 

5.  The  Cooperation  of  Cells  and  Organs,  and  the  Net 
Result.  —  In  the  foregoing  paragraphs  we  have  repeatedly 
spoken  of  the  work  of  the  cells  (and  organs)  as  if  this 
work  were  the  sole  end  and  object  of  their  existence.  If, 
however,  we  turn  back  to  Chapter  IV  and  recall  its  prin- 
cipal lesson,  namely,  that  the  net  result  (resultant)  of  the 
work  of  the  individual  cells  and  organs  of  the  body  is  the 
work  of  the  body  itself,  then  we  shall  perhaps  begin  to 
realize  that  no  matter  how  important  to  the  cell  or  to  its 
neighbors  individual  cell  work  may  be,  this  is  even  more 
important  as  an  indispensable  constituent  in  the  work  of 
the  body  as  a  whole.  Any  one  can  observe  the  human 
mechanism  actively  working  at  home  or  in  field  or  factory; 
or  running,  leaping,  or  sleeping ;  or  engaged  in  professions 
or  business  or  trade ;  but  only  the  physiologist  realizes 
how  these  most  various  acts  are  merely  the  net  result  of 
the  lives  and  activities  of  myriads  of  individual  cells, — lives 
in  which  the  cells  not  only  minister  and  are  ministered  unto, 
but  also  work  together  both  for  mutual  benefit  and  for  the 
benefit  of  the  body  as  a  whole. 

Least  of  all  do  we  commonly  realize  that  it  is  not  merely 
the  operation,  but  the  cooperation,  —  not  merely  the  work, 
but  the  combined,  and  especially  the  harmonious  or  orderly, 
work  of  the  myriad  cells  of  the  body,  united  as  they  are 
into  one  grand  army,  —  that  underlies  the  effective  work 
of  the  human  mechanism  as  a  whole.  To  the  study  of  the 
mechanism  of  this  cooperation  we  may  now  proceed. 


CHAPTER  VII 

THE  ADJUSTMENT  OR  COORDINATION  OF  THE  WORK  OF 
ORGANS   AND   CELLS 

A  great  physiologist  once  said,  "  Science  is  not  a  body 
of  facts ;  it  is  the  explanation  of  facts."  Some  of  the 
most  important  chapters  of  science  are  those  which  seek  to 
explain  facts  so  well  known  and  obvious  that  we  are  apt 
to  forget  that  they  need  explanation.  When  anything  irri- 
tates the  lining  of  the  nasal  cavity  we  sneeze ;  when  it 
irritates  the  larynx  we  cough ;  when  it  irritates  the  exposed 
surface  of  the  eyeball  we  wink.  These  three  facts  are 
well  enough  known;  but  it  is  safe  to  say  that  any  one 
considering  the  matter  for  the  first  time  would  find  it  dif- 
ficult to  explain  how  it  comes  about  that  anything  going 
"  down  the  wrong  way  "  does  not  make  us  sneeze  or  wink, 
but  sets  us  to  coughing.  The  answer  to  the  general  ques- 
tion thus  raised  is  the  subject  of  this  chapter,  which  con- 
siders the  adjustment  of  the  work  of  the  individual  cells 
and  organs  of  the  body,  each  to  do  its  work  at  the  proper 
time,  and  so  to  play  its  due  part  in  the  work  of  the  organism 
as  a  whole. 

The  more  we  think  of  it,  the  more  wonderful  does  this 
fact  of  adjustment  appear.  The  millions  of  living  cells  are 
in  a  way  individual  units,  and  communities  of  individuals 
do  not  invariably  work  together.  Let  us  compare  the  human 
body  in  this  respect  with  bodies  or  groups  of  men  or  boys. 
In  a  game  of  football  each  team  is  a  body  of  eleven  indi- 
viduals, and  each  individual  is  assigned  to  a  definite  posi- 
tion to  do  definite  things  as  occasion  arises.  Theoretically, 

70 


COORDINATION  71 

under  given  conditions  of  the  game  it  is  the  work,  or  func- 
tion, of  the  "  left  tackle  "  to  prevent  a  certain  player  of  the 
opposing  side  from  making  a  certain  play.  But  there  is 
always  a  doubt  whether  he  will  do  this  thing,  or  "  lose  his 
head  "  and  do  something  else,  leaving  his  man  free  to  do 
what  he  pleases.  In  the  latter  case  that  organism  which 
we  call  a  football  eleven  would  act  very  much  as  the  human 
organism  would  act  if  it  were  to  wink  and  not  cough  when 
a  foreign  body  lodges  on  the  lining  membrane  of  the  larynx. 

Evidently  we  have  something  here  to  explain.  Why 
are  the  actions  of  the  body  purposeful,  that  is,  adapted  to 
accomplish  the  right  thing  at  the  proper  time?  and  in  the 
more  complicated  actions,  how  is  the  work  of  the  different 
units  —  the  organs  and  the  cells  —  adjusted,  or  coordinated; 
that  is  to  say,  how  is  each  one  made  to  do  its  proper  share  of 
the  work?  Let  us  begin  with  the  study  of  a  very  simple 
action,  —  that  of  winking. 

1.  Winking  is  caused  by  the  contraction  of  muscle  fibers 
which  run  transversely  across  the  eyelid  in  a  curved  course. 
As  they  are  attached  most  firmly  at  the  regions  A  and  B 
(Fig.  36),  their  shorten- 
ing straightens  their 
arched  course  and  so 
brings  the  two  edges  of 
the  eyelid  into  contact. 
The  work  of  this  muscle 
is  obviously  purposeful, 

for  the  wink  takes  place 

i          .,  i    n  i         FIG.  36.  The  muscular  mechanism  of 

when  the  eyeball  needs  winking 

protection ;  it  is  also  co- 
ordinated, since  the  act  is  executed  by  a  number  of  fibers 
working  together,   for  if  only  those  of  the  lower  eyelid 
were  to  contract  the  lids  could  not  be  closed. 

The  muscle  fibers  which  work  together  to  produce  the 
wink  do  not  originate  their  own  activity.  They  merely  do 


THE  HUMAN  MECHANISM 


what  they  are  stimulated  to  do  by  the  nervous  impulse, 
which  acts  upon  the  muscular  fuel  substance  somewhat  as 
a  fuse  acts  upon  a  charge  of  gunpowder.  Even  the  amount 
of  contraction  is  determined  by  the  strength  of  the  nervous 
impulse,  a  strong  impulse  producing  greater  contraction 
than  a  weak  impulse.  In  health  the  muscle  fibers  are  the 


FIG.  37.  Cross  section  of  a  nerve 

Showing  five  bundles  of  nerve  fibers  bound  together  by  connective  tissue  con- 
taining a  few  blood  vessels.  On  the  right  are  shown  four  fibers  more  highly 
magnified,  the  dark  center  being  the  axon,  around  which  is  the  white  or 
fatty  sheath,  both  axon  and  fatty  sheath  being  inclosed  within  the  fine 
membrane,  the  neurilemma.  Compare  Fig.  38 

obedient  servants  of  the  nerves,  and  if  they  act  in  a  pur- 
poseful and  coordinated  manner,  it  is  because  the  nerves 
stimulate  them  to  act  in  this  way.  The  explanation  of  pur- 
poseful and  coordinated  action  must  therefore  be  sought 
not  in  the  muscles  but,  behind  these,  in  the  nervous  system, 
to  the  study  of  which  we  now  turn. 

2.  Structure  of  a  Nerve.  —  A  nerve,  like  a  muscle,  may 
be  separated  into  long  fibers  (Fig.  38)  which  are  bound  to- 
gether by  connective  tissue  containing  blood  vessels,  lymph 


COORDINATION 


73 


spaces,  and  lymphatics.    The  nerve  fiber,  which  is  the  essen- 
tial part  of  the  nerve,  just  as  the  muscle  fiber  is  of  the  muscle, 
differs  somewhat  in  structure  in  different  nerves  ;  it  gener- 
ally consists  of  a  central  threadlike 
core  surrounded  by  a  fatty  sheath, 
the  latter  being,  therefore,  shaped 
like    a   hollow   cylinder,  —  which, 
however,  is  interrupted  at  intervals 
of  about  one  millimeter, —  and  both 
of  these  are  enveloped  in  a  delicate 
membrane  comparable  to  the  sarco- 
lemma  of  the  muscle  fiber.    Such 
fibers  are  from  about  -g^Vo  *°  TTo'o 
of  an  inch  in  diameter  (compare  the  FlG  38    Four  nerve  fibers 
diameter  of  a  muscle  fiber,  p.  35).  (highly  magnified) 

There  are,  however,  nerve  fibers  E,  node  of  Ranvier  at  which 
which  have  no  fatty  sheath,  and       th^fatty  sheath  is  discon- 

others  which  are  destitute  of  mem- 
brane.  The  essential  part  of  the  fiber  is  the  threadlike  por- 
tion in  the  center;   this  is  never  absent  from  nerves  and 
is  known  as  the  axon,  or  axis  cylinder. 

3.  The  Axon  of  a  Nerve  Fiber  is  a  Branch  of  a  Nerve 
Cell.  —  By  suitable  methods  these  axons  may  be  traced 
along  the  nerve  of  which  they  form  part,  and  even  into  the 
brain  and  spinal  cord ;  it  is  then  found  that  they  pursue 
an  uninterrupted  course  and  ultimately  become  continuous 
with  the  cytoplasm  of  a  nerve  cell.  Nerve  cells  are  found 
in  the  brain,  in  the  spinal  cord,  in  enlargements  (ganglia)  on 
certain  nerves,  and  even  alone  in  the  connective  tissue  of 
many  organs  of  the  body,  as  the  heart,  the  intestine,  etc. 
By  far  the  greater  number  are  in  the  brain  and  spinal  cord, 
and  in  some  cases  the  axons  to  which  they  give  rise  are  of 
very  considerable  length ;  those  of  the  muscles  of  the  foot, 
for  example,  reach  from  cells  in  the  sacral  region  of  the 
spinal  cord  to  the  extremity  of  the  foot.  Such  fibers  would 


74 


THE  HUMAN  MECHANISM 


be  over  a  yard  long  and  less  than  j^Vo  °^  an  "lcn  wide, 
and  we  may  regard  the  cell  whose  main  portion  is  in  the 
sacral  cord  as  sending  out  a  branch,  or  process,  from  this 
region  to  the  foot. 

Furthermore,  recent  investigations  have  led  to  the  gener- 
ally accepted  conclusion  that  each  axon  is  a  part  of  only 


FIG.  39.  Four  nerve  cells 

A  and  (7,  from  the  cerebellum ;  B,  from  the  gray  matter  of  the  spinal  cord ; 
D,  from  the  cerebrum;  a,  the  axon.  The  cells  A  and D  are  stained  so  that 
the  main  body  and  the  dendrites  (p.  76)  are  a  uniform  black ;  B  and  C,  so 
as  to  show  the  nucleus  and  cytoplasm 

one  nerve  cell;  a  single  cell  may  give  off  more  than  one 
axon,  but  the  axon  is  never  connected  with  more  than  one 
nerve  cell.  Of  these  cells  and  of  their  connections  with 
nerve  fibers  we  can  get  a  more  definite  picture  by  an  exam- 
ination of  the  structure  of  the  spinal  cord. 

4.  Structure  of  the  Spinal  Cord.  —  When  the  vertebral 
canal  is  opened  a  whitish   cord  is  found  within  it, — the 


COORDINATION 


75 


Dorsal  Root 


Dorsal 

Ganglion 


spinal  cord,  —  from  each  side  of  which  arise  thirty-one  pairs 
of  nerves,  or,  in  general,  one  pair  for  each  vertebra.  One 
nerve  of  each  pair  arises  on  the  ventral  side  of  the  cord, 
the  other  on  the  dorsal  side.  These  nerves  are  known  as 
the  ventral  and  dorsal  nerve  roots1  respectively.  On  the 
dorsal  nerve  root  some  Dorsal 

distance  from  the  cord 
there  is  a  slight  enlarge- 
ment, or  ganglion.  Just 
outside  this  ganglion 
the  two  roots  unite, 
and  from  their  union 
nerves  pass  to  the  skin, 
the  muscles,  the  blood 
vessels,  the  viscera,  etc.  Dor8al 

The  spinal  cord  itself 
in  cross  section  shows 
a  darker  central  core, 
known  as  the  gray  mat- 
ter, surrounded  by  an 
outer  lighter  portion, 
the  white  matter.  The 
white  matter  consists  essentially  of  nerve  fibers  which  run 
lengthwise  of  the  cord  and  here  and  there  send  branches 
into  the  gray  matter  ;  it  may  be  regarded  as  a  large  nerve. 
The  gray  matter,  on  the  other  hand,  contains  a  mesh  of 
fibers,  and  in  addition  numerous  nerve  cells.  There  is  the 
same  difference  everywhere  between  the  white  and  gray 
matter  of  the  nervous  system  ;  the  arrangement  in  the 
brain  is  not  so  simple  as  in  the  cord,  but  here  also  the 
white  matter  consists  of  fibers  running  from  one  part  of 
the  nervous  system  to  another,  while  the  masses  of  gray 
matter  always  include  collections  of  nerve  cells. 


Dorsal  Root 

Ganglion 

EIG.  40.  The  origin  of  the  dorsal  and 
ventral  nerve  roots  of  a  segment  of 
the  spinal  cord 


1  The  older  anatomical  terms,  and  those  even  to-day  more  generally 
used,  are  anterior  and  posterior  instead  of  ventral  and  dorsal. 


76  THE  HUMAN  MECHANISM 

5.  Fibers  of  the  Ventral,  or  Anterior,  Nerve  Root.  — 
These  fibers  may  be  traced  into  the  spinal  cord.  It  is  then 
found  that  the  nerve  cells  from  which  they  arise  lie  in  the 
gray  matter  in  the  immediate  neighborhood  of  the  root  to 
which  they  belong ;  i.e.  the  fibers  of  the  roots  do  not  come 
from  higher  or  lower  parts  of  the  cord,  or  from  the  brain. 
It  has  also  been  found  that  when  they  are  stimulated 
they  throw  muscles  into  contraction  and  produce  effects 
on  the  blood  vessels  and  glands,  but  they  do  not  give 
rise  to  sensations,  nor  produce  other  effects  in  the  cord 
itself.  In  other  words,  the  fibers  of  the  ventral  root  conduct 
impulses  from  the  cells  of  the  spinal  cord  outward ;  they 
do  not  conduct  impulses  from  outside  into  the  spinal  cord. 
Hence  they  are  known  as  efferent  fibers  (Latin,  ex,  out  of ; 
ferre,  to  carry). 

The  nerve  cells  from  which  these  fibers  arise  consist  of 
a  mass  of  cytoplasm  around  the  nucleus  and  of  one  or 
more  outgrowths  of  this  cytoplasm,  usually  more  or  less 
branched.  These  outgrowths,  or  processes,  divide  and  sub- 
divide, ultimately  forming  in  the  gray  matter  exceedingly 
fine  terminal  branches  like  those  of  a  tree  in  the  air.  Such 
processes  are  known  as  dendrites  (Greek,  dendron,  a  tree). 
The  nerve  cells  in  question  have  numerous  dendritic  proc- 
esses ;  in  other  nerve  cells  there  may  be  but  one,  and 
still  others  possess  no  dendritic  processes  at  all.  In  all 
cases  the  general  appearance  of  the  cell  depends  largely 
upon  the  number  and  manner  of  branching  of  these  proc- 
esses. Thus  it  happens  that  nerve  cells  differ  from  one 
another  in  appearance  just  as  a  Lombardy  poplar,  an  oak, 
an  elm,  and  a  maple  differ,  although  all  show  the  funda- 
mental characteristics  of  a  tree  (Fig.  39). 

In  subsequent  portions  of  this  work  it  is  unnecessary 
for  us  to  go  into  the  details  of  the  form  of  the  nerve 
cells  to  any  extent;  the  student  need  only  understand 
henceforward  that  nerve  cells  consist  of  a  central  mass  of 


COOBDINATION  77 

nucleated  cytoplasm  from  which  proceed  outgrowths,  or 
processes,  which  are  of  two  kinds,  —  (1)  those  which  become 
axons  of  nerve  fibers,  and  which  form  an  essential  part  of 
all  nerve  cells  ;  and  (2)  the  dendrites,  which  are  usually  but 
not  always  present.  The  whole  structure,  including  the 
central  cell  body  with  its  dendrites  and  axons,  is  an  ana- 
tomical unit, —  a  cell.  To  this  entire  cell  the  term  neurone 
is  given.  The  neurone  is  the  cellular  unit  of  the  nervous 


FIG.  41.  Semidiagrammatic  longitudinal  section  of  a  ganglion 
of  the  dorsal  (posterior)  root 

system,  just  as  the  muscle  fiber  is  the  cellular  unit  of  the 
muscle,  and  the  gland  cell  of  the  gland. 

6.  Fibers  of  the  Dorsal,  or  Posterior,  Roots.  —  The  ven- 
tral roots,  as  we  have  seen,  are  entirely  efferent  in  func- 
tion; that  is,  they  conduct  impulses  only  away  from  the 
spinal  cord.  The  dorsal,  or  posterior,  roots,  on  the  other 
hand,  are  found  to  be  essentially  afferent  (Latin,  ad,  to; 
ferre,  to  carry),  i.e.  they  carry  impulses  from  outside  toward 
and  into  the  spinal  cord.  This  is  shown  by  the  fact  that  when 


78  THE  HUMAN  MECHANISM 

these  roots  are  destroyed  by  disease  muscles  can  still  be 
thrown  into  contraction,  glands  will  still  secrete,  etc.,  — i.e. 
there  is  no  interference  with  efferent  impulses,  —  but  no 
sensations  are  received  from  the  part  of  the  body  to  which 
these  nerves  are  distributed ;  pinching  the  skin  is  not  felt ; 
the  flesh  may  be  burned  and  its  owner  be  entirely  uncon- 
scious of  it.  Since  these  results  never  follow  destruction 
of  the  ventral  roots,  we  must  conclude  that  impulses  enter 
the  cord  solely  by  the  dorsal  roots  precisely  as  they  leave 
the  cord  solely  by  the  ventral  roots. 

It  has  been  stated  above  (p.  75)  that  there  is  a  ganglion 
on  the  dorsal  root.  Microscopic  study  of  this  ganglion 
shows  that  the  fibers  of  the  dorsal  root  pass  through  it,  and 
that  each  fiber  gives  off  at  right  angles  to  itself  a  branch 
which  becomes  continuous  with  a  pear-shaped  nerve  cell 
of  the  ganglion.  These  cells  have  no  other  processes.  We 
may  express  the  relation  between  the  pear-shaped  cells  of 
the  ganglion  and  the  fibers  of  the  dorsal  root  by  saying 
that  the  single  axon  from  the  main  cell  body  divides  into 
two  in  the  ganglion,  one  branch  passing  outward  to  the 
periphery,  and  the  other  centrally  into  the  spinal  cord 
(Fig.  41). 

7.  Endings  of  the  Peripheral  Branches  of  the  Neurones 
of  the  Dorsal   Root   in  Sense   Organs.  —  The  peripheral 
branch  ultimately  ends  in  some  "  sense  organ,"  one  of  the 
most  important  of  which,  so  far  as   the  spinal  nerves  are 
concerned,  is  the  skin.    The  eye,  the  ear,  the  nose,  the 
mouth,  are  examples  of  other  sense  organs,  and  they  all 
contain  the  peripheral  endings  of  afferent  neurones.    Each 
is  sensitive  to  some  special  influence  from  without,  as  the 
eye  to  light,  the  ear  to  sound,  etc.  ;  and  when  stimulated 
they  start  nerve  impulses  moving  inwards  along  the  nerves 
toward  the  brain  or  cord. 

8.  Ending  in  the  Spinal  Cord  of  the  Central  Branch  of 
the  Neurones  of  the  Dorsal  Root.  —  The  other  or  central 


COORDINATION  79 

branch  passes  into  the  spinal  cord.  It  does  not,  however, 
like  the  neurones  of  the  ventral  root,  there  become  con- 
tinuous with  the  nerve  cells  of  the  gray  matter,1  but  divides, 
on  entering  the  cord,  into  an  ascending  and  a  descending- 
branch  ( Fig.  42),  each  of  which  runs  for  a  longer  or 
shorter  distance  in  the  white  matter  of  the  cord.  Indeed, 
many  of  the  ascending  branches  extend  as  far  anteriorly 
as  the  lower  parts  of  the  brain.  As  shown  in  the  figure, 
these  branches  give  off  at  right  angles  to  themselves  sub- 
branches,  the  collaterals,  each  of  which  enters  the  gray 
matter  and  ends  there  by  breaking  up  into  a  tuft  of 
extremely  fine  fibrils,  the  synapse.  The  synapse  is  in  close 
proximity  to  and  possibly  in  a  kind  of  anatomical  con- 
tinuity with  the  dendrites,  or  the  main  body  of  nerve  cell 
of  the  gray  matter.  Each  afferent  neurone,  then,  is  a  cell 
whose  main  body  is  in  the  ganglion  of  the  dorsal  root, 
and  whose  branches,  or  arms,  reach  out,  one  of  them  to  a 
peripheral  sense  organ  and  the  other  to  the  gray  matter 
of  the  spinal  cord  and  brain,  where  they  end  in  synapses. 
By  means  of  the  synapses  the  afferent  neurone  excites  or 
stimulates  other  neurones. 

9.  Anatomical  Relation  of  Afferent  to  Efferent  Neurones. 
—We  may  now  put  together  what  we  have  learned  about  the 
neurones  of  the  ventral  and  those  of  the  dorsal  root ;  we  then 
obtain  a  plan  like  that  shown  in  Fig.  42,  and  such,  in  prin- 
ciple at  least,  represents  the  manner  in  which  the  afferent 
neurone  is  brought  into  relation  with  efferent  neurones. 

Afferent  and  efferent  fibers  enter  and  leave  portions  of 
the  brain  in  much  the  same  way,  although  the  separation 
into  ventral  and  dorsal  roots  is  not  obvious.  We  may 
therefore  take  the  above  scheme  as  typical  of  the  relation 
between  these  two  kinds  of  neurones,  —  those  of  the  brain 
as  well  as  those  of  the  cord. 

1  It  is,  as  has  already  been  pointed  out  (p.  78),  part  of  a  nerve  cell 
in  the  ganglion  of  the  dorsal  root. 


80 


THE  HUMAN  MECHANISM 


10.  Application  of  These  Facts  of  Structure  in  the  Ex- 
planation of  Purposeful  and  Coordinated  Action.  —  The 
diagram  in  Fig.  43  readily  explains  why  the  sudden  ap- 
pearance of  an  object  in  front  of  the  eye  causes  us  to 
wink  and  not  cough;  that  is  to  say,  it  explains  the  pur- 
poseful character  of  this  so-called  reflex  action.  The  for- 
mation of  the  image  of  the  object  on  the  retina,  a  sense 


A  B 

FIG.  42.  Eelation  of  afferent  (a/)  to  efferent  (ef)  neurones  of  the 
spinal  cord 

In  A  the  single  afferent  neurone  branches  into  six  collaterals,  each  of  which 
ends  in  a  synapse  around  an  efferent  cell.  In  B  the  connection  is  made 
through  the  agency  of  the  cell  a,  as  explained  in  Section  13 

organ,  starts  impulses  along  the  fibers  of  the  afferent  optic 
nerve ;  these  fibers  extend  into  the  brain,  and  their  synap- 
ses end  around  and  stimulate  those  efferent  nerve  cells 
which  stimulate  the  muscles  of  the  eyelid.  The  action  is 
purposeful  because  the  fibers  of  the  optic  nerve  end 
around  these  cells,  and  not  around  those  which,  for  exam- 
ple, innervate 1  the  muscles  which  open  the  mouth  or  flex 
the  finger  (Fig.  43). 

1  i.e.  supply  with  nerve  fibers. 


COOBDINATION 


81 


- <4 


Our  diagram  also  gives  the  basis  of  coordination, — 
the  combination  of  the  work  of  different  muscle  fibers  in 
orderly  harmonious  action.  The  system  of  collaterals  on 
the  central  branch  of  the 
afferent  neurone  is  obviously 
a  mechanism  to  combine  the 
action  of  the  efferent  neu- 
rones in  this  way.  The  dia- 
gram also  gives  a  clew,  at 
least,  to  the  explanation  of 
another  element  of  coordi- 
nation :  when  two  or  more 
muscles  work  together  to  ac- 
complish a  given  act,  one  of 
the  muscles  usually  works  FIG.  43.  Diagram  of  the  nervous 
harder  than  another;  not  mechanism  by  which  a  wink  is  pro- 
, -,  ,  duced  by  the  sudden  appearance 

only    must    they    work    to-        Of  an  object  in  front  of  the  eye 
gether,    but    the    amount    of    r,  afferent  neurone  of  the  optic  nerve ; 
force    exerted  by  each  must         m»  »*',  m",  m"',  efferent  neurones 

be  adjusted  to  the  needs  of  ' the  muscles  oi  the  eyelid 
the  movement  as  a  whole.  This  adjustment  is  most  proba- 
bly effected  by  differences  in  the  connection  of  the  synapses 
with  their  cells;  thus  those  muscles  which  contract  most 
forcibly  are  innervated  by  neurones  whose  dendrites  and 
main  cell  body  come  into  more  intimate  contact  with  the 
synapses  of  the  afferent  neurone ;  or  the  number  of  fibrils 
of  the  synapse  may  be  greater  in  their  case  than  in  the 
others.  These,  however,  are  only  possibilities;  the  whole 
subject  requires  further  elucidation. 

11.  Definition  of  Reflex  Action.  —  An  action  such  as  we 
have  just  been  studying  is  known  as  a  reflex  l  action.  By 

1  The  word  literally  suggests  the  idea  of  reflection  from  the  afferent  to 
the  efferent  neurones,  as  light  is  reflected  from  a  surface;  but  the  stu- 
dent has  already  learned  enough  to  understand  that  efferent  impulses  are 
something  more  than  mere  mechanical  reflections,  or  rebounds,  of  afferent 
impulses. 


82 


THE  HUMAN  MECHANISM 


this  we  mean  an  action  called  forth  by  the  more  or  less 
direct  action  of  afferent  upon  efferent  neurones,  and  without 
the  intervention  of  the  will.  The  afferent  neurone  may  be 
stimulated  by  some  external  agent,  such  as  light,  heat, 

sound,  pressure,  etc., 

j,  or  by  some  condition 

within  the  body  it- 
self, as  when  dis- 
eased or  abnormal 
conditions  of  the 
stomach  or  some 
other  organ  induce 
vomiting. 

It  is  a  common 
error  to  suppose  that 
all  actions  which  are 
not  called  forth  by 
the  will  are  reflex. 
The  essential  feature 
of  a  true  reflex  is  the 
more  or  less  direct 
action  of  the  afferent  impulses  on  efferent  neurones,  and 
not  merely  its  non volitional  character.  There  are,  in  fact, 
involuntary  actions  in  which  the  efferent  neurones  are 
directly  stimulated  not  by  afferent  neurones  but  by  the 
condition  of  the  blood,  or  in  other  ^ways.  Such  actions 
are  not  reflex,  though  they  may  be  either  involuntary 
or  unconscious,  or  both.  They  are  known,  in  general,  as 
automatic  actions,  and  we  shall  meet  examples  of  them  as 
we  proceed  with  the  study  of  the  various  functions  of 
the  body. 

12.  Actions  resulting  from  Stimulation  by  the  Will.  — 
A  wink  is  not  always  a  reflex  action.  We  can  wink  "  on 
purpose,"  or,  otherwise  expressed,  a  wink  may  be  called 
forth  by  the  will,  and  entirely  apart  from  the  sudden 


FIG.  44.  Diagram  of  the  nervous  mechanism 
represented  in  Fig.  43,  with  the  addition 
of  the  neurone  6  (see  Section  12) 


COORDINATION 


83 


appearance  of  some  object  in  front  of  the  eye.  Here  the 
muscles  of  the  eyelid  act  in  exactly  the  same  manner  as  in 
a  reflex  wink,  which  means  that  they  are  stimulated  in 
the  same  way  by  the 
same  efferent  neurones. 
Thus  far  the  mechanism 
is  the  same  in  the  two 
cases,  but  the  source  of 
stimulation  of  the  effer- 
ent neurones  must  be 
different. 

In  later  chapters  of 
this  book  we  shall  bring 
forward  evidence  to 
show  that  the  exercise 
of  the  will  (volition)  re- 
quires the  cooperation  FIG.  45.  The  nervous  mechanism  shown 


in  Fig.  44,  with  the  addition  of  the 
afferent  neurone  c,  from  the  cornea 
(see  Section  12) 


of  the  highest  portion 
of  the  brain  or  cerebrum. 
Nerve  cells  in  the  gray 
matter  of  the  cerebrum  send  off  axons  which  pass  down- 
ward to  those  portions  of  the  brain  and  spinal  cord  from 
which  the  motor  or  efferent  neurones  arise ;  with  the  neu- 
rones of  these  nerves  they  make  exactly  the  same  kind  of 
connections  (collaterals  and  synapses)  as  are  made  by  the 
afferent  fibers  from  the  retina  which  excite  the  reflex  (see 
Fig.  44,  in  which  b  is  the  cerebral  neurone). 

The  collaterals  and  synapses  of  the  cerebral  neurone 
(which,  it  will  be  observed,  is  entirely  confined  to  the  cen- 
tral nervous  system)  simply  duplicate  those  of  the  afferent 
neurone;  hence  the  two  neurones  produce  the  same  result. 

There  is,  however,  still  a  third  way  in  which  winking 
may  be  stimulated.  When  the  cornea  of  the  eye  begins  to 
dry,  a  reflex  wink  spreads  tears  over  the  eyeball.  In  this 
case  we  have  to  deal  with  a  second  reflex,  the  afferent 


84 


THE  HUMAN  MECHANISM 


neurones  being  not  those  in  the  optic  nerve,  but  those  in 
what  is  known  as  the  trigeminal,  the  sensory  nerve  of  the 
cornea.  Our  scheme  thus  becomes  that  shown  in  Fig.  45. 
13.  The  "  Master  "  Neurone.  —  The  multiplication  of 
collaterals  and  arborizations  which  this  scheme  involves 
would  seem  to  be  largely  avoided  by  the  presence  of  a 

third  neurone  between 
those  which  stimulate 
the  action  and  the  effer- 
ent neurones  which 
directly  act  on  the  mus- 
cles (Fig.  46). 

In  this  way,  when  a 
wink  is  produced, 
whether  from  the  cere- 
brum or  from  the  retina 
or  from  the  cornea,  the 
single  cell  a  is  stimu- 
lated; and  this  in  turn 
stimulates  the  groups 
of  efferent  neurones 

which  immediately  innervate  the  muscles  of  the  eyelids. 
Many  of  the  nerve  fibers  of  the  cord  and  brain  belong  to 
neurones  which  perform  the  same  function  as  that  attrib- 
uted to  the  cell  a  in  our  diagram.  They  are  entirely  con- 
fined to  the  brain  or  cord,  and  group  together  those 
efferent  cells  which  by  working  together  produce  a  coor- 
dinated action. 

The  organization  of  the  nervous  system  is,  in  fact,  much 
like  that  of  a  large  manufacturing  establishment.  The 
nerve  cells  which  send  axons  to  the  muscles,  glands, 
blood  vessels,  etc.,  may  be  compared  with  the  operatives,  each 
with  his  special  task  to  perform ;  over  these  are  foremen, 
or  "bosses,"  from  whom  they  take  their  orders,  or,  in 
physiological  language,  who  stimulate  them  to  do  their 


FIG.  46.  The  "master"  neurone 


COORDINATION  85 

work,  and  who  would  correspond  to  cells  like  a  in  Fig.  46. 
The  foremen  in  turn  receive  orders,  now  from  one  depart- 
ment of  the  establishment,  now  from  another,  as  the  work 
of  their  operatives  is  needed  in  making  one  or  the  other  of 
the  products  offered  for  sale.  So  the  "  master  "  neurones 
receive  stimuli  from  the  brain,  or  from  afferent  nerves,  as 
the  needs  or  the  desires  of  the  organism  as  a  whole  require 
their  activity.  The  comparison  is  instructive  and  may 
easily  be  carried  out  in  greater  detail  by  the  student 
himself. 

By  this  time  the  student  has,  no  doubt,  made  the  dis- 
covery that  a  mere  wink  is  after  all  an  extremely  complex 
event;  and  yet  winking  is  very  simple  compared  with 
many  of  our  actions,  such,  for  example,  as  throwing  a 
stone.  Here  not  only  muscles  which  produce  motion  at 
the  shoulder,  elbow,  wrist,  and  finger  joints  are  called  into 
play,  but  also  muscles  which  maintain  the  erect  position 
and  balance  of  the  body  as  a  whole.  The  entire  mechan- 
ism involved  baffles  the  imagination  to  conceive  ;  and  yet 
any  boy  can  perform  the  act.  He  can  do  it,  however, 
because  his  motor  neurones  are  grouped  together  into  a 
perfectly  well-organized  army  which  executes  at  once  the 
bidding  of  its  commander-in-chief,  —  the  will. 

We  have  given  in  this  chapter  only  the  merest  glimpse 
into  the  complexity  of  one  part  of  the  wonderful  human 
mechanism.  No  watch,  no  machine  which  man  has  ever 
invented  or  constructed  can  for  a  moment  compare  with 
this  living  machine  in  complexity  or  in  perfection.  Yet, 
like  all  machines,  this  one  can  be  abused,  it  can  get  out  of 
order,  it  can  even  break  down.  And  we  have  already 
learned  enough  to  understand  why  this  is  so.  Some  neu- 
rones may  be  injured  by  overwork,  or  may  degenerate 
from  disuse ;  indulgence  in  stimulants  or  narcotics  may 
poison  the  governing  nerve  cells ;  above  all,  constant  fail- 
ure to  lead  a  normal  life  may  deprive  these  precious  cells 


86  THE  HUMAN  MECHANISM 

of  their  sole  means  of  repair.  The  human  body  is  a  machine 
designed  for  use,  even  for  hard  use,  and  it  thrives  upon 
right  use;  but  it  is  a  machine  too  delicate  and  too  com- 
plex to  be  abused  with  impunity. 

When  one  thinks  of  the  hundreds,  perhaps  thousands,  of 
movements  which  the  body  makes,  and  of  the  combination 
of  these  movements  into  definite  actions  or  work,  and  then 
reflects  that  the  muscle  fibers  which  execute  any  movement 
are  thrown  into  orderly  contraction  by  nerve  cells  which 
are  themselves  commanded  by  higher  nerve  cells ;  that 
these  in  turn  are  marshaled,  as  it  were,  by  still  higher  cells 
when  the  separate  movements  they  evoke  are  to  be  com- 
bined into  a  still  more  complicated  action,  —  one  begins  to 
appreciate  the  complexity  of  the  organization  of  the  nervous 
system.  The  number  of  the  nerve  cells  is  measured  by 
hundreds  of  thousands,  and  their  efficiency  in  directing  the 
working  organs  of  the  body,  so  as  to  meet  the  demands  of 
life,  depends  not  only  upon  the  integrity  of  the  neurones 
but  also  upon  the  perfection  of  their  organizations,  i.e. 
their  grouping  into  squads,  companies,  regiments,  brigades, 
divisions,  and  corps,  ready  to  yield  instant  and  obedient 
response  to  the  command  of  the  higher  officers  of  the  will, 
or  to  the  signals  of  those  pickets,  —  the  sense  organs  and 
their  afferent  neurones,  —  which  everywhere  guard  the  out- 
posts and  give  information  of  the  need  for  action. 

Moreover,  this  army  of  neurones,  like  any  other  army, 
becomes  efficient  by  work,  by  drilling,  by  practice,  even  by 
battle.  Like  the  soldiers  of  a  regular  army,  the  neurones 
may  be  overworked,  and  the  efficiency  of  the  military  body 
may  suffer  thereby  ;  but  they  may  also  work  too  little  ;  the 
perfection  of  their  development,  of  their  organization,  de- 
pends on  the  practice  they  get  with  reasonable  activity. 
To  this  point  we  shall  return ;  but  meantime  the  student 
can  safely  make  the  application  for  himself.  Such  com- 
parison and  such  application  are  not  only  instructive  but 


COOEDINATION  87 

intensely  practical  in  their  bearing  upon  the  affairs  of 
everyday  life,  —  upon  that  right  conduct  of  life  which  is 
the  first  duty  of  every  man,  every  woman,  every  child. 

14.  The  Activity  of  the  Individual  Neurones.  —  We  have 
thus  far  emphasized  the  grouping  of  the  neurones  and  their 
organization  into  an  army,  and  have  said  but  little  of  what 
takes  place  in  the  individual  neurone  during  activity.  We 
have  once  or  twice,  to  be  sure,  spoken  of  the  discharge  of 
a  nervous  impulse  from  the  cell,  and  the  passage  of  a  nerv- 
ous impulse  along  the  fiber,  but  have  avoided  any  discus- 
sion as  to  what  these  nervous  actions  really  are.  To  go  into 
these  matters  in  detail  would  take  us  too  far  afield.  We 
may  simply  say  that  there  is  every  reason  for  believing  that 
the  activity  of  the  nerve  cell,  like  that  of  the  muscle  fiber, 
involves  chemical  changes  in  the  cell,  —  a  consumption  of 
stored  material  and  the  production  of  carbon  dioxide,  water, 
acids,  etc.  It  has  been  shown,  in  fact,  that  the  nucleus  and 
cytoplasm  actually  change  their  form  and  appearance  after 
work,  somewhat  as  gland  cells  do  after  prolonged  activity. 
The  general  physiology  of  the  individual  nerve  cells  would 
thus  be  similar  to  that  of  the  individual  muscle  fibers  and 
gland  cells  described  in  Chapter  IV. 

The  nerve  impulse  which  passes  along  the  axon  of  the 
nerve  fiber,  on  the  other  hand,  appears  to  be  a  physical 
change  rather  than  an  explosive  chemical  decomposition, 
—  comparable,  for  example,  rather  to  the  conduction  of  a 
current  of  electricity  through  a  salt  solution  than  to  the 
progress  of  chemical  change  along  a  fuse  of  gunpowder. 

The  nervous  system  participates  in  every  act  of  human 
life,  and  as  we  study  further  the  functions  of  the  body  we 
shall  constantly  extend  our  knowledge  of  its  working.  The 
present  chapter  is  intended  to  give  only  some  indispensa- 
ble preliminary  information.  To  other  of  the  more  impor- 
tant functions  and  relations  of  the  nervous  system  we  shall 
return  in  Chapters  XIV  and  XV. 


CHAPTER  VIII 
ALIMENTATION  AND  DIGESTION 

A.   THE  SUPPLY  OF  MATTER  AND  POWER  TO  THE 
HUMAN  MACHINE 

1.  Power  and  the  Materials  for  Repair  supplied  sepa- 
rately to  Lifeless  Machines.  —  Living  and  lifeless  machines 
are  alike  in  that  worn-out  parts  must  be  renewed  and  that 
power  must  be  supplied  to  do  work.    In  the  lifeless  machine 
these  two  requirements  are  supplied  separately.    A  factory 
and  its  equipment  of  machinery  are  kept  in  repair  and 
enlarged  (grow)  by  means  of  bricks,  lumber,  steel,  belting, 
new  pieces  of  machinery,  etc.,  which  are  brought  into  the 
building;  while  the  power  which  runs  the  machinery  comes  in 
quite  separately  as  fuel,  or  water  power,  or  electric  power. 

2.  Power  and  the  Materials  for  Growth  and  Repair  sup- 
plied to  the  Human  Machine  in  the  One  Form  of  Foods.  - 
With  the  human  mechanism  this  is  not  so.    Materials  for 
growth  and  repair,  and  power  for  running,  are  introduced 
from  without  not  separately,  but  together,  both  being  sup- 
plied in  the  one  form  of  food.    As  it  does  its  life  work  the 
human  mechanism,  like  a  lifeless  machine,  not  only  con- 
sumes power  but  its  parts  deteriorate,  and  it  is  the  double 
function  of  the  food  we  eat  to  make  good  this  double  loss. 
Some  foods  possibly  serve  only  as  means  of  power  ;  others 
merely  make  good  the  loss  of  essential  parts  of  the  mechan- 
ism ;  while  still  others  may  serve  both  purposes. 

3.  Food  as  a  Source  of  Power.  —  Experiment  and  experi- 
ence alike  prove  that  foods  are  the  source  of  power  for 
work.    Bread,  butter,  starch,  sugar,  beef,  and  the  like  may 

88 


ALIMENTATION  AND  DIGESTION  89 

be  dried  and  then  burned  as  fuel,  giving  power  to  an 
engine.  The  occasional  use  of  Indian  corn  or  wheat  for 
fuel,  in  the  West,  the  employment  of  hams  and  bacon  as 
fuel  by  steamers  short  of  coal,  the  explosion  of  flour  dust 
in  mills,  and  similar  phenomena  further  illustrate  by  the 
teachings  of  experience  the  fact  that  these  foods  are  rich 
in  energy,  or  power. 

When  we  say  that  the  food  must  supply  power  to  the 
body  we  mean  that  the  power  which  it  contains  must  be 
available  to  the  body.  A  lump  of  coal  may  be  a  source  of 
power,  as  is  shown  by  its  use  in  a  locomotive ;  but  a  lump 
of  coal  would  be  of  no  use  as  food,  because  the  body  has 
no  such  means  of  burning  it  as  has  the  engine.  Again, 
nitroglycerin  contains  chemical  elements  needed  in  the 
food  ;  but  although  when  exploded  in  a  dynamite  cartridge 
it  may  furnish  power  enough  to  shatter  heavy  armor  plate, 
its  energy  is  not  available  to  the  body. 

Thus,  to  recapitulate,  food  (a)  makes  good  the  loss  of 
living  substance  in  the  body,  (b)  it  supplies  material  for 
growth,  and  (c)  it  supplies  power  for  the  work  which  the 
body  is  to  do.  It  also  performs  one  more  important  func- 
tion which  will  be  more  clearly  understood  hereafter; 
for  (d)  by  its  oxidation  food  provides  the  heat  usually 
required  to  keep  up  the  body  temperature.  The  detailed 
consideration  of  this  subject,  however,  must  be  postponed 
to  Chapter  XII. 

4.  Chemical  Composition  of  Foods  ;  Nutrients.  —  The  hu- 
man race  has  learned  by  long  experience  that  certain  things 
do,  and  that  other  things  do  not,  meet  the  demands  of  the 
body  for  food.  Perhaps  no  animal  uses  so  many  different 
materials  as  man  in  satisfying  sensations  of  hunger  and 
thirst.  Some  foods  are  taken  from  the  animal  and  some 
from  the  vegetable  kingdom,  and  their  variety  is  greatly 
increased  by  special  modes  of  preparation.  But  however 
numerous  the  foods  from  which  we  prepare  the  dishes 


90  THE  HUMAN  MECHANISM 

served  at  different  meals,  chemical  analysis  shows  that  the 
essential  constituents  of  all  foods  belong  to  a  compara- 
tively small  number  of  chemical  groups.  These  classes,  or 
groups,  may  be  called  nutrients ;  and  as  all  the  members 
of  the  same  group  undergo  practically  the  same  processes 
of  digestion,  and  perform  similar  functions  in  nourishing 
the  body,  it  will  be  equally  accurate  and  more  convenient, 
in  treating  of  this  part  of  physiology,  to  speak  of  the  dif- 
ferent nutrients,  and  not  of  beef,  mutton,  fish,  eggs,  bread, 
milk,  butter,  etc. 

From  the  point  of  view  of  digestion  the  most  important 
nutrients  are  the  proteids,  the  albuminoids,  the  carbohy- 
drates, the  fats,  the  inorganic  salts,  and  water;  and  the 
student  must  at  this  point  become  thoroughly  familiar  with 
what  is  meant  by  these  fundamental  terms. 

5.  The  Group  of  Proteids.  —  We  may  obtain  a  working 
idea  of  what  a  proteid  is  by  recalling  some  of  the  foods  in 
which  proteid  preponderates  or  is  easily  seen.  Such  foods 
are  the  white  of  egg,  the  lean  of  tender  meat  (muscle  fibers), 
the  curd  of  milk,  the  tenacious  gluten  of  wheat.  Proteids 
also  exist  in  relatively  large  quantities,  though  not  so  readily 
seen,  in  yolk  of  egg,  beans,  peas,  oats,  and  other  grains. 

Proteids  contain  carbon,  hydrogen,  nitrogen,  oxygen,  and 
sulphur.  Some  of  them  contain  phosphorus,  and  some  iron. 
Chemically  they  are  exceedingly  complex  substances,  the 
proteid  molecule  consisting  of  a  very  large  number  of  atoms. 
We  cannot  write  the  chemical  formula  for  proteid  as  we 
write  H2O  for  water  ;  we  know  the  proportions  by  weight 
in  which  the  elements  are  combined,  but  we  have  not  yet 
discovered  how  many  atoms  of  each  element  there  are  in  the 
molecule.  It  should  be  noted  that  the  proteids  are  the  most 
important  nutrients  which  contain  nitrogen  and  sulphur. 

Many  proteids  readily  become  insoluble.  Examples  of 
this  are  the  hardening  of  the  white  of  egg  or  the  lean  of 
meat  by  cooking,  and  of  the  casein  or  curd  of  milk  by 


ALIMENTATION  AND  DIGESTION  91 

rennet  or  "  junket  tablets."  This  change  is  known  as  coagu- 
lation, and  most  of  our  proteid  food  is  eaten  after  having 
been  coagulated  in  the  process  of  cooking. 

Proteids  occur  only  within  the  living  cells  of  plants  and 
animals,  or  as  the  products  of  these  living  cells.  They  form, 
as  we  shall  more  clearly  see  later,  the  basis  of  the  living 
cell,  and  are  constantly  disintegrating  within  the  cell  into 
simpler  substances.  Hence  there  is  a  constant  cellular  loss 
of  proteid,  which  in  the  animal  body  can  be  made  good 
only  from  proteid  in  the  food.  Plants,  on  the  other  hand, 
have  the  power  of  manufacturing  proteids  from  sugars 
and  certain  mineral  salts,  which  latter  supply  the  needed 
nitrogen  and  sulphur.  The  plant  kingdom  is,  therefore,  in 
the  long  run  the  sole  source  of  proteid  food  for  animals, 
for  while  some  animals  (carnivores)  get  their  proteid  entirely 
by  eating  the  flesh  of  other  animals,  the  latter  (herbivorous 
animals)  in  turn  have  obtained  their  proteid  from  plants. 

6.  The  Group  of  Albuminoids.  —  Closely  related  to  the 
proteids  in  chemical  composition,  and  indeed  derived  from 
them,  is  a  group  of  bodies  known  as  the  albuminoids. 
Physiologically  they  cannot  entirely  replace  proteids  in 
our  diet,  but  they  can  replace  them  partially.    The  most 
important  of  these  substances  in  use  as  food  is  the  fibrous 
connective  tissue,  whose  fibers  in  the  uncooked  state  con- 
tain the  albuminoid  substance  collagen,  which  by  heating 
in  the  presence   of  water  is   converted  into  the   closely 
related  gelatin.    Other  albuminoids  are  of  minor  practical 
importance. 

7.  The  Group  of  Carbohydrates. — Within  the  cells  which 
make  up  a  potato  or  a  grain  of  wheat  one  often  sees  a  large 
number  of  granules  of  starch.    This  starch  was  derived 
originally,  with  the  aid  of  the  sunlight  and  the  green  parts 
of  the  plant,  from  carbon  dioxide  obtained  from  the  air,  and 
water  absorbed  by  the  roots.     It  is  a  typical  carbohydrate 
(a  term  shortly  to  be  explained)  and  constitutes  the  chief 


92  THE  HUMAN  MECHANISM 

carbohydrate  food  of  the  world.  It  is  readily  changed  into 
the  other  members  of  the  same  group,  —  the  gums,  or  dex- 
trines,  and  the  sugars.  The  latter  are  often  formed  in  the 
plant  itself,  as  in  sugar  maples,  sugar  cane,  sugar  beets,  and 
in  such  fruits  as  apples,  pears,  and  peaches. 

Carbohydrates  are  compounds  of  the  elements  carbon, 
hydrogen,  and  oxygen,  the  last  two  usually,  but  not  always, 
occurring  in  the  proportions  of  two  atoms  of  hydrogen  to 
one  of  oxygen.  The  carbohydrates  actually  used  as  foods 
belong  for  the  most  part  to  three  closely  related  groups. 
The  members  of  the  simplest,  the  monosaccharides,  have 
the  formula  C6H12O6,  and  the  most  common  are  grape  sugar 
and  fruit  sugar ;  the  second  group  includes  cane  sugar  and 
milk  sugar,  each  of  which  has  the  formula  C12H22On  ;  the 
third  group  includes  the  gums  and  starches  which  have 
the  formula  (C6H10O5)n  where  n  is  some  factor  which  mul- 
tiplies the  numbers  6,  10,  and  5.  Thus  while  the  exact 
formula  for  starch  is  not  known  it  is  safe  to  say  that  n  is 
as  large  as  25.  This  would  make  the  above  formula 
^I50^-25o^i25-  The  molecule  of  starch  therefore  contains 
a  very  large  number  of  atoms  of  carbon,  hydrogen,  and 
oxygen,  but  no  nitrogen. 

8.  The  Group  of  Fats.  —  Fats  are  familiar  to  us  in  such 
forms  as  butter,  lard,  olive  oil,  and  the  fat  of  meat.    Like 
the  carbohydrates  they  are  compounds  of  carbon,  hydrogen, 
and  oxygen,  although  the  oxygen  is  always  present  in  small 
quantities.     The  formula  for  one  of  the  fats  is  C51H98O6, 
and  this  composition  is  typical  of  all  of  them. 

Fats  may  be  split  up  into  certain  acids  (fatty  acids)  and 
glycerin  (C3H8O5),  and  when  treated  with  alkalis,  like 
caustic  soda  or  caustic  potash,  they  form  soaps.  They  are 
insoluble  in  water.  Like  the  carbohydrates  they  contain 
no  nitrogen. 

9.  Oxidizable  and   Nonoxidizable   Nutrients.  —  All  the 
above  nutrients  may  and  do  combine  with  oxygen  within 


ALIMENTATION  AND  DIGESTION  93 

the  cells  of  the  body,  although  the  way  in  which  this 
chemical  union  is  brought  about  is  one  of  the  unsolved 
problems  of  physiology.  While  all  of  the  nutrients  may 
be  burned  after  being  dried,  such  combustion  requires  a 
high  temperature.  Within  the  body  they  are  not  only 
burned  (i.e.  combined  with  oxygen)  at  a  temperature  rarely 
exceeding  39°  C.  (100°  F.),  but  they  undergo  oxidation 
while  in  a  moist  state,  or  even  in  solution.  However  this 
oxidation  may  be  effected  within  the  cell,  there  can  be  no 
doubt  that  it  yields  the  heat  for  keeping  the  body  warm 
and  the  power  for  its  work. 

The  remaining  groups  of  nutrients,  the  inorganic  salts 
and  water,  are,  for  the  most  part,  not  oxidized  in  the  body. 

10.  The  Groups  of  Inorganic  Salts  and  Water. — These 
nonoxidizable  nutrients  are  absolutely  necessary  for  the 
proper  nourishment  of  the  body.    Their  presence  in  the 
blood  and  lymph,  and  in  the  living  cells,  is  indispensable 
to  the  processes  of  life.    The  salts  are  taken  in  small  quan- 
tities, partly  as  salt  itself,  partly  as  portions  of  the  various 
foods  we  eat.    During  growth  they  furnish  much  of  the 
mineral  matter  of  bones,  and  since  the  body  is  daily  losing 
salt,  it  is  necessary  that  salt   be  supplied  in  the  food. 
Salts,  however,  are  not  acted  on  to  any  large  extent  in  the 
alimentary  canal  by  the  processes  of  digestion ;  they  are 
largely  absorbed  in  the  same  form  as  eaten.    Hence  they 
do  not  concern  us  at  present  to  the  same  extent  as  do  the 
oxidizable  nutrients  which  generally  have  to  be  chemic- 
ally changed,  or  digested,  before  they  can  be  absorbed  for 
use  in  the  body.    And  the  same  thing  is  true  of  water. 

11.  Indigestible  Material  in  Food.  —  When  we  say  that 
a  food  is  digestible  we  mean  that  when  taken  info  the  ali- 
mentary canal,  if  not  already  in  solution,  it  is  chemically 
acted  upon  by  the  digestive  juices  so  as  to  be  dissolved 
and  made  capable  of  being  absorbed  into  the  blood.    The 
greater   part  of   the   food   we  eat   consists  of  digestible 


94 


THE  HUMAN  MECHANISM 


substances,  but  many  foods  contain  a  certain  amount  of 
indigestible  material,  and  some  a  very  considerable  amount. 
The  most  conspicuous  example  of  such  material  is  cel- 
lulose, a  member  of  the  same  group  of  carbohydrates  to 
which  starch  belongs.  It  occurs  in  almost  all  vegetable 
foods,  and  since  in  the  human  alimentary  canal  cellulose 


FIG.  47.  Part  of  the  seed  of  the  bean 


FIG.  48.  Section  of  potato 


Showing  the  larger  starch  and  the  finer      Showing  starch  granules  inclosed 
proteid  granules  inclosed  within  the  within  the  cellulose  cell  walls 

cellulose  cell  walls 

is  for  the  most  part  unaffected,  it  cannot  be  absorbed  and 
necessarily  forms  an  important  part  of  the  feces.  Other 
indigestible  substances  are  the  outer  skin  of  animals,  e.g. 
the  skin  of  fowls,  and  certain  portions  of  the  connective 
tissue  of  meat. 

12.  Animal  and  Vegetable  Foods.  —  The  classification  of 
foods  into  animal  and  vegetable  not  only  describes  the 
origins  of  foods  from  the  two  great  kingdoms  of  living 
things  but  also  defines  important  differences  between  them 
with  reference  to  digestion.  These  differences  may  be 
summed  up  as  follows. 

Animal  foods  are  generally  rich  in  proteids  and  poor  in 
carbohydrates,  while  vegetable  foods  are  generally  poor  in 
proteids  and  very  rich  in  carbohydrates,  especially  starch. 
In  the  second  place,  animal  foods  contain  relatively  little 
indigestible  material,  while  vegetable  foods,  as  they  occur 


ALIMENTATION  AND  DIGESTION 


95 


in  nature,  contain  large  amounts  of  indigestible  cellulose. 
In  the  third  place,  the  digestible  materials  of  vegetable 
foods  (the  proteids,  carbohydrates,  and  fats)  are  often  con- 
tained within  a  plant  cell  which  is  surrounded  by  a  cellu- 
lose membrane  impermeable  to  the  digestive  juices  ;  before 
they  can  be  digested  this  membrane  must  be  ruptured  in 
one  way  or  another.  In  the  case  of  many  animal  foods,  on 
the  other  hand,  especially  meat  and  fat,  the  cells  (muscle 
fibers  and  fat  cells)  which  contain  the  essential  nutrients 
are  held  together  by  connective  tissue  made  up  largely  of 
fibers  of  an  albuminoid  nature.  These  fibers  are  soluble 
in  the  juices  of  the  stomach,  in  which  the  cellulose  which 
holds  together  the  vegetable  foods  is  insoluble.  The  full 
importance  of  these  differences  will  be  evident  before  we 
have  finished  the  study  of  digestion. 

13.  Composition  of  Some  Common  Foods.  —  The  follow- 
ing table  gives  the  percentage  composition  of  some  of  the 
more  common  foods. 


WATER 

PROTEID 

STARCH 

SUGAR 

FAT 

SALTS 

Bread  .... 

37 

8 

47 

3 

1 

2 

Wheat  flour  .     . 

15 

11 

66 

4.2 

2 

1.7 

Oat  meal  .     .     . 

15 

12.6 

58 

5.4 

5.6 

3 

Rice     .... 

13 

6 

79 

0.4 

0.7 

0.5 

Peas     .... 

15 

23 

55 

2 

2 

2 

Potatoes    .     .     . 

75 

2 

18 

3 

0.2 

0.7 

Milk     .... 

86 

4 



5 

4 

0.8 

Cheese 

37 

33 

— 

— 

24 

5 

Lean  beef      .     . 

72 

19 

— 

— 

3 

1 

Fat  beef   .     .     . 

51 

14 





29 

1 

Mutton      .     .     . 

72 

18 

— 



5 

1 

Veal     .... 

63 

16 

— 

— 

16 

1 

White  fish     .     . 

78 

18 





3 

1 

Salmon 

77 

16 

— 

— 

5.5 

1.5 

Egg      .... 
Butter.     .     .     . 

74 
15 

14 

= 

— 

10.5 

83 

1.5 
3 

96  THE  HUMAN  MECHANISM 

14.  The  Process  of  Alimentation.  —  Before  corn,  wheat, 
meat,  vegetables,  and  other  food  materials  can  be  taken 
into  the  body  and  made  to  yield  up  to  it  the  material  and 
power  which  they  contain,  they  must,  in  most  cases,  undergo 
various  preparatory  or  preliminary  processes  or  treatments 
which  shall  make  them  easier  or  better  to  eat,  or  more  at- 
tractive. The  most  familiar  of  these  processes  is  cooking, 
but  it  is  by  no  means  the  only  one.  In  the  case  of  animal 
food  the  animal  must  be  captured,  if  wild,  or  raised,  if 
domesticated.  It  must  be  killed,  skinned,  dressed,  cut  up, 
and  the  meat  in  many  cases  "  ripened "  by  keeping,  or 
"  cured  "  by  smoking,  salting,  drying,  or  corning.  So,  also, 
with  plant  food,  such  as  cereals,  vegetables,  fruits,  nuts, 
and  the  like ;  these  must  first  be  found,  if  wild,  or  grown, 
if  domesticated.  They  must  then  be  separated  from  the 
rest  of  the  plant,  —  threshed,  if  wheat,  rye,  oats,  or  barley  ; 
husked  and  shelled,  if  corn ;  dug  up  or  removed  from  the 
earth,  if  vegetables  like  potatoes,  celery,  radishes,  or  let- 
tuce. Fruits  and  nuts  must  be  separated  or  picked  from 
vine  or  tree  ;  milk,  drawn  from  animals  ;  and  even  salt, 
water,  and  condiments  like  mustard  and  pepper,  separated 
from  the  earth  or  the  sea  or  from  plants.  After  collection 
and  further  preparation  by  winnowing,  grinding,  or  clean- 
ing, elaborate  cooking  is  applied  to  many  forms  of  food 
before  it  is  put  upon  the  table;  and  even  then,  at  the  last 
moment  before  it  is  eaten,  a  further  separation,  as  of  meat 
from  bone,  must  be  made  either  by  the  carver  or  by  the 
eater  himself. 

To  this  entire  process  of  the  supply  and  preparation  of 
food  for  eating  the  term  "alimentation"  may  be  conven- 
iently applied.  Reflection  will  show  that  it  is  largely  a 
process  of  food  refining,  the  principal  result  being  a  con- 
centration of  the  nutrients  at  every  step.  It  is  also  a 
separation  of  the  comparatively  useful  from  the  compara- 
tively worthless  (as  food) ;  and  just  here,  and  in  these  two 


ALIMENTATION  AND  DIGESTION  97 

points,  —  refining  and  the  separation  of  good  from  poor  mate- 
rials, —  we  may  recognize  a  true  process  of  digestion,  but 
one  external  rather  than  internal :  a  refining  in  the  field, 
the  mill,  and  the  kitchen  rather  than  in  the  stomach ;  in 
the  environment  rather  than  within  the  organism. 

15.  Digestion,  External  and  Internal.  —  The  word  "  diges- 
tion "  comes  from  the  Latin  dis  and  gerere,  to  tear  apart 
or  separate.    A  "  digest  "  of  the  laws  of  any  state  or  coun- 
try is  a  compact,  concentrated  statement  of  those  laws 
from  which  the  unimportant  parts  have  been  separated 
and  omitted.    Now  one  of  the  principal  objects  and  func- 
tions of  digestion  is  to  separate  the  nutritious  and  therefore 
important   part  of  the  food   from   the    innutritions    and 
therefore   unimportant.    Another  and  no  less  important 
function  is  to  prepare  this  food  that  it  may  be  more  readily 
absorbed  and  utilized  by  the  body  proper.    A  survey  of 
the  processes  of  alimentation  and  digestion  shows  that  all 
are  tributary  to  these  ends,  and  that  we  may  with  advan- 
tage consider  alimentation  (including  cooking)  as  a  kind  of 
external  digestion,  which  applies  to  the  food  a  treatment 
not  only  preliminary  to  but  also  preparatory  for  ordinary 
or  internal  digestion.    Digestion  as  commonly  understood 
thus  becomes  only  the  last  part  of  a  much  longer  process. 

16.  The  Alimentary  Canal  a  Digestive  Laboratory.  - 
Many  animals  low  in  the  scale  of  life,  and  all  plants,  are 
destitute  of  those  tubes  running  through  the  body,  which 
are    called    alimentary    canals,    but    all    animals    (except 
some  parasites)  above  the  lowest  are  provided  with  such 
tubes  or  canals.    These  are  really  no  more  than  places,  or 
spaces,  in  which  food  is  prepared  for  absorption,  and  they 
always  closely  resemble  chemical  laboratories  or  refineries 
in   which  raw   materials   are    altered    or   refine.d  by   the 
action  of  chemical  reagents.    The  laboratories  are  of  sev- 
eral kinds,  —  mouth,  stomach,  intestine, — and  the  reagents 
correspondingly  different,  —  salivary  juice,  gastric  juice, 


98  THE  HUMAN  MECHANISM 

pancreatic  juice,  bile,  intestinal  juice,  etc.  To  the  detailed 
consideration  of  internal  or  ordinary  digestion  we  may  now 
proceed. 

B.  DIGESTION  IN  THE  MOUTH.    THE  TEETH.    ENZYMES 

17.  Digestion.  —  The  food  upon  which  the  body  depends 
for  its  supplies  of  matter  and  power  enters  the  body  proper 
through  the  lining  membranes  of  the  alimentary  canal  into 
which  it  is  received  through  the  open  mouth.    Inasmuch, 
however,  as  these  lining  membranes  contain  no  visible  open- 
ings, the  food  must  somehow  soak,  or  "  diffuse,"  through 
them,  and  in  order  to  do  this,  solid  foods  must  be  greatly 
changed  and  prepared  for  absorption  by  being  brought  into 
solution. 

But  this  is  not  all.  Some  foods  are  soluble  but  still  not 
suitable  for  the  use  of  cells  and  tissues.  Common  cane 
sugar  (saccharose),  for  example,  is  very  soluble  ;  but  before 
it  can  be  used  by  the  cells  it  must  be  converted  into  the 
closely  related  but  somewhat  different  sugars  known  as 
dextrose  and  levulose.  Again,  proteid  food  is  absolutely 
indispensable  for  all  animals,  but  ordinary  proteids  pass 
through  membranes  like  those  of  the  alimentary  canal 
with  great  difficulty ;  they  are,  therefore,  before  absorp- 
tion, turned  into  other  and  more  diffusible  bodies  known 
as  peptones.  Starches  and  fats  also  require  special  treat- 
ment to  make  them  available  or  acceptable  for  absorption. 
We  shall  begin  our  study  of  the  processes  of  digestion 
with  those  which  take  place  in  the  mouth,  and  these  are 
the  crushing  or  grinding  of  the  food  by  the  teeth  and  its 
mixture  with  saliva. 

18.  Structure  of  a  Tooth.  —  A  tooth  has  three  parts,  - 
the  crown,  or  exposed  portion  ;  the  neck,  a  narrow  con- 
striction at  the  edge  of  the  gum  ;  and  the  root,  or  roots,  by 
which  the  tooth  is  fixed  in  the  jawbone  (see  Fig.  138). 


ALIMENTATION  AND  DIGESTION 


99 


The  tooth  consists  of  a  hard  body  surrounding  a  central 
pulp  cavity,  filled  with  a  loose  connective  tissue  containing 
blood  vessels  and  nerves,  which  enter  the  pulp  cavity  by  a 
minute  opening  on  the  tip  of  each  root.  Elsewhere  the 
pulp  is  surrounded  by  the  hard  parts  of  the  organ,  which 
consist  of  three  different  tissues. 
Immediately  surrounding  the  pulp, 
both  in  the  root  and  in  the  crown, 
is  the  dentine,  which  makes  up  the 
main  bulk  of  the  tooth  ;  this  is  a 
hard  structure,  containing  some  65 
per  cent  of  mineral  matter,  chiefly 
carbonates  and  phosphates  of  cal- 
cium. It  is  channeled,  as  shown  in 
the  figure,  by  minute  canals,  the 
dentinal  tubules,  which  run  into  the 
pulp  cavity.  In  the  root  the  dentine 
is  covered  with  cement,  which  is  vir- 
tually bone  in  structure  and  compo- 
sition. In  the  crown,  or  that  part  FlG-  49-  Section  of  an  in- 
of  the  tooth  not  covered  by  the  gum,  00^  After  Spal~ 

the  dentine  is  covered  with  enamel, 
the  hardest  substance  in  the  body. 
This  contains  in  the  adult  from  95 
to  97  per  cent  of  very  insoluble  mineral  matter,  and  is  the 
protective  covering  of  the  tooth.  In  structure,  enamel  con- 
sists of  columns,  hexagonal  in  section,  set  together  so  as 
to  form  a  practically  impenetrable  mosaic  covering.  It  is 
thus  admirably  fitted  to  protect  the  dentine,  and  indeed 
the  whole  tooth,  from  mechanical  injury,  from  chemical 
erosion,  and  from  bacterial  action. 

19.  Care  of  the  Teeth.  —  Too  much  stress  can  hardly  be 
laid  on  the  preservation  of  the  teeth.  Apart  from  consid- 
erations touching  personal  appearance,  the  teeth  are  of 
great  importance  in  masticating  the  food.  Mastication,  or 


A,  enamel;  B,  dentine;  C, 
gum;  D,  pulp;  E,  jaw- 
bone; F,  cement 


100  THE  HUMAN  MECHANISM 

chewing,  is  one  of  the  many  acts  of  digestion,  and  when 
the  power  of  chewing  is  impaired,  the  efficiency  of  the 
whole  digestive  process  is  to  that  extent  lessened  ;  other 
portions  of  the  alimentary  tract,  especially  the  stomach, 
must  then  do,  as  far  as  possible,  what  should  have  been 
done  by  the  teeth  ;  digestion  is  hindered ;  some  kinds  of 
food  are  never  properly  digested,  and  the  opportunity  for 
bacterial  decomposition  of  the  food  is  greatly  increased, 
because  of  the  prolonged  exposure  of  the  food  to  bacte- 
rial action.  Besides,  there  is  always  the  possibility  that  a 
decaying  tooth  will  cause  the  formation  of  an  abscess  (an 
ulcerated  tooth),  often  a  most  painful  and  sometimes  a 
dangerous  thing. 

20.  Decay  of  the  Teeth  is  usually,  if  not  always,  due  to 
the  action  of  bacteria,  which  produce  acids  by  fermenting 
food  particles  in  the  mouth.  These  acids  dissolve  away 
the  lime  salts  of  the  enamel  and  the  dentine  ;  the  enamel, 
however,  is  acted  upon  very  slowly  and  with  great  diffi- 
culty ;  so  long  as  it  is  intact,  the  underlying  dentine, 
which  is  dissolved  much  more  readily,  is  protected ;  but  if 
for  any  reason  the  enamel  becomes  worn  away,  its  absence 
should  be  made  good  by  filling  the  tooth,  thereby  prevent- 
ing access  of  foreign  substances,  and  especially  of  destruc- 
tive bacteria,  to  the  dentine. 

The  action  of  bacteria  upon  the  enamel  is  favored  by 
the  formation  of  a  hard  deposit  known  as  tartar,  a  mix- 
ture of  lime  salts  precipitated  from  the  saliva,  and  espe- 
cially apt  to  be  deposited  between  the  lower  teeth  and  in 
the  neighborhood  of  the  gums.  Sometimes  tartar  is  even 
deposited  under  the  gums,  in  which  case  it  is  inaccessible 
to  the  action  of  a  brush.  Because  of  this  tartar  crust,  bac- 
teria and  the  acids  which  they  produce  are  not  properly 
rubbed  away  by  the  movements  of  tongue  and  cheeks, 
and  hence  the  importance  of  its  artificial  removal.  This  is 
greatly  facilitated  by  using  a  tooth  powder  which  contains 


ALIMENTATION  AND  DIGESTION  ,         1Q1 

some  substance,  like  precipitated  chalk,  not  hard  enough 
to  injure  the  enamel  but  exerting  friction  enough  to  break 
up  the  deposit.  At  least  once  or  twice  a  week  a  good 
tooth  powder  should  be  used  in  cleaning  the  teeth.  At 
other  times  the  brush  and  water  are  sufficient.  After  using 
powder,  indeed  always  after  brushing  the  teeth,  the  mouth 
cavity  should  be  very  thoroughly  rinsed  out. 

It  is,  however,  very  difficult,  and  at  times  impossible, 
to  remove  the  tartar  entirely  by  the  use  of  powder  and 
brush.  For  this  reason  the  teeth  should  be  examined  by 
a  dentist  at  least  once  a  year,  the  accumulated  tartar 
thoroughly  removed,  and  the  teeth  polished.  In  this  way 
the  beginnings  of  decay  are  detected  and  measures  can  be 
taken  to  prevent  its  further  progress.  Further  advice  as 
to  the  care  of  the  teeth  can  and  should  be  obtained  from  a 
good  dentist. 

Decay  of  the  teeth  caused  by  bacteria  is  also  prevented 
by  removing  as  far  as  possible  the  food  supply  of  these 
organisms,  to  whose  growth  and  activity  nothing  is  more 
favorable  than  particles  of  food  between,  or  otherwise  in 
contact  with,  the  teeth.  The  ideal  plan  is  to  brush  the 
teeth  with  water  and  rinse  out  the  mouth  after  each  meal ; 
in  most  cases  this  is  perhaps  more  than  is  required  ;  it 
may  be  suggested,  however,  that  the  teeth  should  be 
brushed  at  night  as  well  as  in  the  morning,  and  that 
brushing  them  at  night  accomplishes  more  toward  re- 
straining bacterial  action  than  does  brushing  them  in  the 
morning. 

By  thus  exercising  care  the  teeth  may  usually  be  pre- 
served in  efficiency  until  old  age.  The  unpleasant  neces- 
sity of  using  false  teeth  is  almost  always  the  result  of 
neglect  of  the  teeth  in  earlier  life. 

21.  Chemical  Action  of  Saliva.  —  We  have  already  stated 
that  the  main  function  of  the  saliva  is  to  lubricate  the 
food  and  so  aid  mastication  and  swallowing.  But  if  saliva 


.102  THE  BUMAN  MECHANISM 

is  brought  in  contact  with  starch,  a  very  characteristic 
chemical  action  occurs,  and  this  we  must  study  in  some 
detail,  chiefly  because  it  is  typical  of  many  similar  chem- 
ical effects  produced  by  other  digestive  juices.  To  demon- 
strate the  effect  in  question  some  starch  paste  should  be 
prepared.  This  is  not  a  clear  solution  like  salt  or  sugar, 
but  an  opalescent  liquid,  which  does  not  become  clear 
upon  passing  through  an  ordinary  filter.  A  characteristic 
test  for  starch  —  the  blue  color  produced  when  a  few 
drops  of  a  solution  of  iodine  are  added  to  it — may  be 
used  to  detect  its  presence  in  the  course  of  the  following 
experiments. 

EXPERIMENT  I 

Three  tubes  or  small  beakers  containing  starch  paste  are  pre- 
pared ;  one  of  them  should  be  cooled  nearly  to  the  freezing  point  by 
packing  ice  around  it,  and  some  filtered  saliva  should  be  cooled  in 
the  same  way.  A  second  portion  of  the  filtered  saliva  is  to  be  heated 
to  the  boiling  point,  while  a  third  is  to  be  used  without  further 
treatment.  To  the  cooled  starch  paste  add  the  cooled  saliva,  keeping 
the  cooled  mixture  surrounded  by  ice ;  to  the  second  portion  of  the 
starch  paste  add  the  boiled  saliva,  and  to  the  third  the  remain- 
ing portion  of  saliva.  Mere  observation  of  the  three  test  tubes  will 
soon  show  that  while  the  first  two  remain  opalescent,  the  last  soon 
becomes  clear.  After  this  change  has  occurred,  a  little  of  the  third 
mixture  may  be  removed,  diluted  with  water,  and  tested  with  iodine  ; 
the  color  now  produced  is  no  longer  a  pure  blue,  but  purplish,  i.e.  a 
mixture  of  blue  and  red ;  some  minutes  later  the  iodine  test  gives  a  port- 
wine  color,  and  still  later  no  color  at  all.  This  shows  that  under  the 
action  of  the  saliva  the  starch  has  disappeared  from  the  third  test  tube. 
The  intermediate  port-wine  color  is  due  to  the  formation  of  dextrines, 
or  animal  gums,  into  which  the  starch  is  first  transformed ;  but  sub- 
sequently this,  also,  disappears.  Meanwhile,  the  starch  in  the  first 
two  test  tubes  shows  no  change  either  in  its  opalescent  appearance 
or  in. its  original  reaction  with  iodine.  Keep  these  test  tubes,  the 
cooled  one  still  surrounded  by  ice,  for  the  subsequent  examination 
described  in  Experiment  III. 


ALIMENTATION  AND  DIGESTION  103 

EXPERIMENT  II 

Let  us  now  inquire  what  has  become  of  the  starch  in  the  third 
test  tube.  The  solution  looks  clear  and  has  a  sweetish  taste.  More- 
over, if  boiled  with  a  mixture  of  sodium  hydroxide  and  copper  sul- 
phate, it  gives  a  red  precipitate  indicating  the  presence  of  sugar. 
These  simple  tests,  then,  prove  that  saliva  first  changes  starch  into  dex- 
trines  and  subsequently  changes  these  dextrines  into  sugar. 

22.  The  Salivary  Ferment,  or  Enzyme.  —  This  change^  of 
starch  first  into  dextrine  and  then  into  sugar  is  caused  by 
ptyalin,  a  constituent  of  the  saliva  which  belongs  to  the 
general  group  of  enzymes  or  unorganized  ferments.  A 
striking  feature  about  the  action  of  ptyalin  is  the  fact  that 
it  is  not  destroyed  or  consumed  in  producing  the  reaction. 
This  is  unlike  what  usually  occurs  to  substances  taking 
part  in  chemical  reactions.  When  wood  combines  with 
oxygen  in  burning,  both  the  wood  and  the  oxygen  disap- 
pear and  in  their  place  the  products  of  combustion  — 
smoke,  gas,  etc.  —  make  their  appearance.  In  the  salivary 
digestion  of  starch  three  substances  enter  into  the  reaction, 
—  starch,  water,  and  the  enzyme ;  two  of  these,  starch  and 
water,  disappear ;  the  third,  the  enzyme,  remains  over  after 
the  reaction  is  completed  and  continues  capable  of  trans- 
forming new  starch  paste  into  sugar. 

We  shall  repeatedly  meet  with  other  enzymes  (pepsin, 
trypsin,  etc.)  in  our  further  studies  of  digestion. 

EXPERIMENT  III 

Returning  now  to  our  experiments  with  the  starch  paste,  we 
find  that  neither  test  tube  1  nor  2,  whether  tested  for  starch  or  for 
sugar,  shows  any  change.  The  student  should  now  remove  a  por- 
tion of  the  cooled  mixture  and  allow  it  to  rise  to  the  room  tempera- 
ture ;  it  will  soon  be  found  that  the  starch  gradually  changes  into 
sugar,  which  shows  that  the  cold  has  prevented  or  inhibited  the  action 
of  the  enzyme  but  has  not  destroyed  it.  If  mixtures  of  saliva  and 
starch  be  kept  at  different  temperatures,  it  will  be  found  that  as  the 


104  THE  HUMAN  MECHANISM 

temperature  of  the  mixture  approaches  that  of  the  body  (98.6°  F.),the 
more  quickly  is  the  transformation  into  sugar  effected.  The  activity 
of  the  enzyme  is  dependent  upon  the  temperature.  If,  however,  we  use 
still  higher  temperatures,  one  will  eventually  be  found  which  destroys 
the  enzyme.  This  temperature  differs  with  different  enzymes,  but  all 
are  destroyed  before  the  boiling  point  of  water  is  reached ;  and  no 
matter  how  long  we  may  keep  a  mixture  of  starch  and  boiled  saliva, 
no  change  of  starch  into  sugar  will  occur. 

EXPERIMENT  IV 

Still  another  instructive  experiment  may  be  made  upon  the  action 
of  saliva  on  starch.  Prepare  five  or  more  small  beakers  of  starch 
paste  and  add  to  the  first  one  a  drop  of  filtered  saliva,  to  the  second 
two  drops,  to  the  third  three  drops,  and  so  on  ;  then  observe  the  time 
required  in  each  case  for  the  disappearance  of  the  iodine  reaction. 
This  experiment  will  show  that  while  a  very  small  amount  of  saliva 
will  transform  an  indefinite  amount  of  starch  to  sugar,  the  more 
saliva  present  the  more  rapidly  will  the  transformation  occur ;  and 
the  same  thing  is  true  of  all  enzymes. 

EXPERIMENT  V 

Finally,  dilute  some  starch  paste  with  an  equal  volume  of  0.4  per 
cent  hydrochloric  acid  (this  will,  of  course,  make  a  0.2  per  cent  solu- 
tion of  the  acid).  Now  add  a  few  drops  of  saliva.  It  will  be  found 
that  no  reaction  takes  place.  Saliva  will  not  act  in  an  acid  medium 
of  this  strength ;  and  it  can  be  easily  shown  that  it  acts  most  vigor- 
ously in  a  neutral  or  faintly  alkaline  solution.  This  result  is  of  much 
practical  importance,  because  the  gastric  juice  contains  approximately 
0.2  per  cent  of  hydrochloric  acid,  and  may  therefore  be  expected  to 
interfere  with  salivary  digestion. 

While  we  are  eating,  the  food  obviously  stays  too  short 
a  time  in  the  mouth  to  allow  the  conversion  of  any  large 
amount  of  its  starch  into  sugar  before  it  is  swallowed. 
Whatever  actual  work  the  saliva  may  do  in  bringing  about 
this  chemical  change  must  evidently  be  done  chiefly  in  the 
stomach,  and  this  will  be  studied  in  the  next  section. 

We  have  dwelt  at  length  upon  the  enzyme  action  of 
saliva  not  merely  for  its  own  sake  but  rather  because 


ALIMENTATION  AND  DIGESTION  105 

the  behavior  of  the  salivary  juice  is  typical  of  the  action 
of  other  of  the  digestive  juices  and  of  enzyme  action  in 
general.  All  the  other  juices  of  the  alimentary  canal,  with 
the  single  exception  of  the  bile,  contain  enzymes,  and  it 
will  greatly  help  our  understanding  of  the  digestive  action 
of  these  enzymes  if  that  of  the  salivary  enzyme  be  first 
mastered. 

Digestion  in  the  mouth,  then,  consists  first,  of  a  mechan- 
ical process  of  chewing,  by  which  food  is  crushed  or  com- 
minuted ;  second,  of  a  physical  process  of  moistening,  by 
which  dry  foods  are  prepared  for  the  act  of  swallowing ; 
and  third,  of  a  chemical  process,  the  chief  part  of  which  is 
the  conversion  of  starch  into  sugar  by  enzyme  action.1 

1  Those  who  have  studied  the  elements  of  chemistry  will  find  the  fol- 
lowing facts  helpful  to  a  fuller  understanding  of  the  action  of  saliva  on 
starch.  Saliva  first  causes  the  large  molecule  of  starch  to  unite  with 
water,  and  in  so  doing  to  split  up  into  one  or  more  molecules  of  sugar 
and  a  certain  gum  (dextrine).  The  saliva  next  causes  the  gum  or  dex- 
trine thus  formed  to  combine  with  water  and  break  up  into  one  or  more 
molecules  of  sugar  and  a  different  gum,  or  dextrine.  The  repetition  of 
this  process  ultimately  transforms  all  the  dextrine  to  sugar.  The  process 
may  be  conveniently  represented  as  follows. 

STARCH  +  water 
sugar  Dextrine  (1)  +  water 

sugar  Dextrine  (2)  +  water 

sugar  Dextrine  (3)  +  water 


sugar  sugar 

(The  original  substance  is  given  in  capitals  and  the  end  products  in 
italics.)  Thus  in  the  above  diagram  one  molecule  of  starch  by  these  suc- 
cessive processes  of  cleavage  would  yield  five  molecules  of  sugar.  In  point 
of  fact,  one  molecule  of  starch  undoubtedly  yields  much  more  sugar  than 


106  THE  HUMAN  MECHANISM 


C.  DIGESTION  IN  THE  STOMACH 

According  to  popular  ideas  the  stomach  is  the  chief  organ 
of  digestion ;  in  fact,  however,  it  is  an  organ  in  which  the 
food  which  has  been  swallowed  is  temporarily  stored  while 
undergoing  a  preliminary  preparation  for  the  more  import- 
ant digestive  changes  which  are  to  take  place  in  the  intes- 
tine. In  this  preparatory  process,  to  be  sure,  some  of  the 

this.  Assuming  as  we  did  on  page  92  that  the  formula  for  starch  is 
Ci5oH250Oi25,  the  reaction  would  be 

CisoHasoOm  +  25H20  =  25C6H1206. 
starch  water  sugar 

In  reactions  like  the  above,  the  sugar  is  known  as  the  end  product, 
while  the  various  dextrines  are  known  as  intermediate  products.  It  is 
also  obvious  that  this  change  from  starch  to  sugar  involves  a  reduction 
in  size  of  the  molecule ;  one  large  molecule  is  split  up  into  many  smaller 
molecules.  This  reduction  in  size  of  the  starch  molecule  appears  to  be  of 
fundamental  physiological  importance,  in  that  the  carbohydrate  food  thus 
acquires  physical  properties  which  render  it  capable  of  passing  through 
the  lining  membrane  of  the  intestine  in  the  process  of  absorption. 

As  stated  in  the  text,  the  chemical  action  of  the  enzyme  at  first  sight 
appears  very  unlike  the  ordinary  chemical  reaction  where  all  substances 
concerned  disappear  in  contributing  to  the  final  result.  For  example, 

HC1  +  KOH  =  KC1  +  H20. 

Here  for  every  molecule  of  hydrochloric  acid  which  disappears,  one 
molecule  of  potassium  hydrate  likewise  disappears.  The  distinction,  how- 
ever, is  only  apparent.  The  reaction  between  hydrochloric  acid  and  potas- 
sium hydrate  should  be  compared  with  the  reaction  between  starch  and 
water,  not  between  starch  and  saliva  ;  and  there  are  numerous  examples 
in  chemistry  of  this  action  of  a  third  body  in  producing  or  accelerating 
a  chemical  reaction  between  two  other  bodies.  Thus  hydrochloric  acid 
acts  very  slowly,  if  at  all,  at  ordinary  temperatures  on  pure  zinc  ;  if, 
however,  a  drop  or  two  of  platinum  chloride  be  added,  vigorous  union 
takes  place : 

Zn2  +  2HC1  =  2  ZnCl2  +  H2. 

Here  again  the  platinum  chloride  by  its  mere  presence  produces  the 
reaction  without  itself  contributing  anything  to  the  compounds  formed. 
Such  reactions  are  in  general  known  as  catalytic,  and  are  by  no  means 
confined  to  living  things.  Their  frequent  occurrence,  however,  in  the 
chemical  processes  of  animal  and  plant  life  is  a  striking  fact. 


ALIMENTATION  AND  DIGESTION 


107 


food  is  incidentally  changed  into  those  forms  in  which  it 
is  taken  into  the  blood ;  but  this  action  is  incidental  and 
subordinate  to  the  main  function. 

23.  Form  and  Structure  of  the  Stomach.  — ^The  stomach 
is  a  large  pouch  into  which  open  two  tubes,  —  the  oesopha- 
gus (gullet)  toward  the  left  side  and  the  intestine  on  the 
right  (see  Fig.  50).  The  two  regions  into  which  these 


Gall 
Bladder 


Bile  Duct 


Intestine 


(Esophagus 


FromtheLiver 
\      \   Pylorus 


'Pancreatic  Duct 


FIG.  50.  Stomach,  beginning  of  small  intestine,  and  entrance  of 
bile  and  pancreatic  ducts 

During  digestion  the  bile  flows  directly  from  the  liver  into  the  intestine ;  at 
other  times  the  opening  of  the  bile  duct  is  closed,  and  the  bile  passes  into 
the  gall  bladder,  where  it  is  stored 

tubes  open  are  different  in  structure,  and  are  known  as 
the  cardiac  (left)  and  pyloric  (right)  portions  of  the  stom- 
ach ;  the  cardiac  portion  differs  from  the  pyloric  portion 
in  having  greater  diameter  and  thinner  walls.  The  entire 
inner  surface  is  lined  by  a  membrane  some  three  or  more 
millimeters  in  thickness,  crowded  with  comparatively  sim- 
ple glands  which  pour  their  secretion,  the  gastric  juice, 


108 


THE  HUMAN  MECHANISM 


into  the  stomach,  very  much  as  sweat  glands  discharge 

perspiration  on  the  skin  (see  Fig.  51). 

The  glandular  membrane  is  one  of  the  two  principal 

components  of  the  stomach  wall;  the  other  is  the  mus- 
cular or  contractile  tissue  which 
forms  a  second  coat  outside  the 
other  and  closely  united  to  it  by 
connective  tissue  containing  the 
larger  blood  vessels,  lymphatics, 
nerves,  etc.  The  muscular  coat 
is  comparatively  thin  in  the  car- 
diac, and  comparatively  thick  in 
the  pyloric  region,  the  thickening 
in  the  latter  region  being  caused 

FIG.  61.  The  inner  surface  of    chiefly  by  muscle  fibers  circularly 
the  stomach  (magnified  , 

about  20  diameters)  arranged. 

Showing   the   openings  of   the          24«    The    Gastric    Juice.  -  -  The 

glands.  The  lining  glandular   gastric  juice  is  a  clear,  thin  liquid, 

membrane  is  thrown  into     which    containg)    among    other 

things,  about  0.2  per  cent  of  hy- 
drochloric acid  and  two  enzymes,  pepsin  and  rennin.  The 
latter  changes  casein,  one  of  the  proteids  of  milk,  into  an 
insoluble  body,  —  the  curd,  —  which  is  subsequently  acted 
upon  by  other  enzymes  of  the  gastric  and  pancreatic  juices 
in  the  same  manner  as  are  other  proteids.1 

The  other  enzyme  of  the  gastric  juice,  pepsin,  is  of  much 
greater  importance.  It  acts  only  in  the  presence  of  free 
acids,  and  on  entering  the  intestine  is  destroyed  by  the  alka- 
line pancreatic  juice;  hence  its  action  is  confined  to  the 
stomach.  It  produces  no  effect  upon  pure  fats  (although  it 
plays  an  important  part  in  the  digestion  of  adipose  tissue), 
nor  upon  carbohydrates,  such  as  starch  and  sugar.  Its  part 
in  digestion  consists  in  its  action  upon  two  constituents 

1  Rennin  is  the  essential  constituent  of  "junket  tablets,"  which  are 
made  from  the  stomachs  of  calves. 


ALIMENTATION  AND  DIGESTION  109 

of  the  food,  —  the  proteids  and  the  connective  tissue  of 
animal  foods.  The  fibers  of  connective  tissue  are  closely 
related  to  the  proteids  in  chemical  composition,  and  the 
action  of  the  enzyme  is  essentially  the  same  on  both ;  their 
complex  molecules  are  split  up  into  smaller  molecules,  and 
the  new  substances  formed  pass  into  solution  in  the  gas- 
tric juice.  By  thus  dissolving  the  connective  tissue,  which 
holds  together  the  muscle  fibers,  fat  cells,  etc.,  animal  food 
is  considerably  subdivided  and  made  to  present  a  greatly 
increased  surface  to  the  further  action  of  digestive  juices. 
It  is  also  well  to  remember  that  the  gastric  juice  dissolves 
connective  tissue  much  more  rapidly  than  does  any  other 
of  the  digestive  juices ;  and  that  this  action  upon  con- 
nective tissue  is  really  more  important  than  that  upon 
proteids,  although  the  latter  is  usually  more  emphasized. 
Proteids  not  acted  on  in  the  stomach  are  rapidly  digested 
by  the  pancreatic  juice  in  the  intestine ;  connective  tissue, 
on  the  contrary,  escaping  solution  in  the  stomach,  is  dis- 
solved but  slowly  in  the  intestine. 

The  action  of  the  gastric  juice,  as  regards  both  proteids 
and  connective  tissues,  consists  essentially  in  splitting  up 
larger  molecules  into  smaller  ones.  This  recalls  the  action 
of  saliva  in  converting  starch  into  dextrines  and  sugars 
(p.  105).  In  the  case  of  proteids  the  products  formed  are 
known  as  proteases  and  peptones,  substances  readily  soluble 
in  the  gastric  juice. 

The  student  is,  however,  warned  against  supposing  that 
because  gastric  juice  is  able  to  transform  the  proteids  of 
the  food  to  peptones,  it  actually  does  exert  this  action 
upon  all  the  proteid  eaten.  In  point  of  fact,  as  proteid 
foods  are  divided  into  smaller  and  smaller  particles  in 
the  stomach,  they  are  discharged  into  the  intestine  where 
their  digestion  is  completed  by  the  pancreatic  juice.  In 
man  the  pancreatic,  and  not  the  gastric  juice,  is  the  main 
agent  of  proteid  digestion. 


110  THE  HUMAN  MECHANISM 

25.  The  Stomach  at  Work.  —  Having  now  gained  a 
general  idea  of  the  chemical  changes  which  occur  in  the 
stomach,  we  may  next  proceed  to  consider  what  actually 
happens  when  food  enters  that  organ.  And  here  our 
knowledge  has  been  gained  partly  by  examining  the  gas- 
tric contents  at  different  periods  of  digestion,  partly  by 
observing  the  movements  of  the  stomach  by  the  aid  of  the 
Rontgen  rays,1  and  partly  by  other  means. 

As  soon  as  food  enters  the  stomach,  and  even  while  it 
is  still  in  the  mouth,  the  gastric  glands  begin  to  discharge 
the  gastric  juice,  and  continue  to  do  so  during  the  four 
or  more  hours  of  gastric  digestion.  When  the  meal  is  fluid 
or  is  small  in  amount  this  gastric  juice  is  thoroughly 
mixed  with  it;  when,  however,  the  food  is  more  or  less 
solid  and  bulky  only  the  outer  layers,  which  are  in  imme- 
diate contact  with  the  walls  of  the  stomach,  are  mixed 
with  the  juice.  At  least  this  is  true  at  the  cardiac  end ; 
the  cavity  of  the  pyloric  portion  is  so  small,  and  the 
amount  of  movement  there  so  great,  that  all  portions  of 
the  pyloric  contents  are  thoroughly  mixed  with  gastric 
juice ;  in  the  much  larger  cardiac  portion  the  central  mass 
of  the  food  may  receive  no  gastric  juice,  and  thus  remain, 
for  an  hour  or  more  after  the  meal,  neutral  or  alkaline 
in  reaction.  Under  these  circumstances  very  considerable 
amounts  of  starch  may  continue  to  undergo  the  salivary 
digestion  begun  in  the  mouth. 

Any  chemical  action  is  aided  by  agitation,  since  the 
reacting  compounds  are  thus  brought  into  more  intimate 
union;  and  observation  of  the  working  stomach  shows 
that  while  the  cardiac  portion  makes  no  movements,  but 

1  For  this  purpose  food  is  swallowed  which  contains  enough  of  the 
harmless  salt,  bismuth  subnitrate,  to  make  the  stomach  contents  imper- 
vious to  the  Rontgen  rays.  The  changes  produced  in  the  shape  of  the 
food  mass  by  the  corresponding  movements  of  the  walls  of  the  stomach 
are  then  observed  by  means  of  shadows  cast  upon  the  fluorescent  screen. 


ALIMENTATION  AND  DIGESTION 


111 


merely  keeps  up  a  steady  contraction  and  thereby  exerts 
a  moderate  pressure  upon  its  contents,  the  pyloric  portion 
executes,  from  a  very  early  stage  of  digestion  and  through- 
out the  whole  process,  a  series  of  movements  which  gradu- 
ally bring  about  a  thorough  mix- 
ture of  the  contents  and  rub 
down  the  softened  food  into 
smaller  and  smaller  masses. 
These  movements  consist  of 
waves  of  constriction  which  arise 
at  the  beginning  of  the  pyloric 
portion  and  pass  onward  to  the 
pylorus  itself,  a  new  wave  begin- 
ning about  once  every  ten  sec- 
onds, and  consuming  from  thirty 
to  forty  seconds  in  passing  to  the 
pylorus.  Consequently  there  are 
always  two  or  more  slowly  mov- 
ing waves  in  the  pyloric  end  of 
the  stomach  at  one  time.1 

The  pyloric  end  of  the  stomach 
is  thus   the    seat  of  a   combined  FIG.  52.  Outline  of  the  contents 

chemical  and  mechanical  action       ^  the  stomach  of  a  cat  at 

three  stages  of  the  digestion 
on  the  food.   The  vegetable  foods        Of  a  meal  taken  about  11  A.M. 

are   Softened,  while   the   COnneC-    Showing  the  peristaltic  constric- 

tive  tissue  of  the  animal  foods 
is  dissolved  away;  in  addition, 
the  food  is  mixed  with  a  consid- 
erable amount  of  liquid  supplied 
by  the  secretion  of  gastric  juice.  The  contents  of  the 
pyloric  end  of  the  stomach  thus  ultimately  come  to  con- 
sist of  minute  solid  masses  suspended  in  a  liquid,  the 

1  These  movements  of  the  stomach  and  intestine  are  best  shown  in 
zoetrope  figures,  which  may  be  obtained  from  the  Harvard  'Apparatus 
Company,  Boston,  Mass. 


tions  which  pass  over  the 
pyloric  portion,  and  the  diminu- 
tion of  the  quantity  of  food  in 
the  cardiac  end.  (Full  descrip- 
tion given  in  Section  25) 


112  THE  HUMAN  MECHANISM 

consistency  of  the  whole  being  that  of  moderately  thick 
pea  soup.  This  product  of  the  work  of  the  stomach  is 
known  as  chyme. 

26.  The  Expulsion  of  Chyme  into  the  Intestine.  —  The 
openings  of  the  oesophagus  and  intestine  into  the  stomach 
are  usually  closed;  the  former  is  opened,  normally,  only 
during  the  act  of  swallowing,  while  the  latter  opens  at 
irregular  intervals  during  the  process  of  gastric  digestion. 
The  opening  of  the  pylorus  allows  the  waves  of  constric- 
tion moving  over  that  region  of  the  stomach  to  discharge 
the  semifluid  chyme  into  the  intestine.  If,  however,  a 
large  mass  of  solid  food  arrives  and  is  driven  against 
the  walls,  the  pylorus  reflexly  closes,  thus  guarding  the 
entrance  of  the  intestine  from  the  passage  of  food  not  yet 
ready  for  intestinal  digestion.  The  pressure  exerted  by 
the  sustained  contraction  of  the  walls  of  the  cardiac  end 
of  the  stomach  adds  to  the  food  in  the  pyloric  region 
new  portions  from  time  to  time,  and  the  same  combined 
chemical  and  mechanical  process  already  described  is  con- 
tinued until  the  whole  mass  is  reduced  to  chyme  and 
driven  into  the  intestine. 

This  brief  sketch  of  the  working  of  the  stomach  shows 
that  this  organ  serves  the  two  main  functions  of  storing 
the  food  and  of  making  it  more  accessible  to  the  diges- 
tive fluids  of  the  intestine.  When  the  chyme  is  delivered 
to  the  intestine  the  mechanical  difficulties  in  th*e  way  of 
absorption  are  practically  gone  ;  the  surface  of  the  food 
exposed  to  digestive  action  is  now  immensely  increased 
by  its  subdivision,  and  the  work  remaining  for  the  intes- 
tine is  almost  wholly  the  chemical  duty  of  changing  the 
constituents  of  the  chyme  into  substances  which  are  soluble 
and  ready  for  absorption. 

Serious  troubles  arise  when,  for  one  reason  or  another, 
gastric  digestion  goes  wrong,  because  the  subsequent 
processes  of  digestion  are  largely  dependent  upon  the 


ALIMENTATION  AND  DIGESTION 


113 


2    3    456     789    10 


preparation  which  the  food  receives  in  the  stomach.  Gas- 
tric digestion  may  be  impaired  in  one  of  three  ways :  first, 
the  gastric  juice  may  not  be  secreted  in  proper  amount 
or  proper  strength;  or,  second,  the  stomach  may  not 
execute  its  movements  efficiently;  or,  third,  the  gastric 
juice  secreted  may  not  be  able  to  get  at  the  food  readily, 
owing  to  improper  cooking  or  insufficient  mastication.  The 
study  of  the  conditions  which  produce  these  troubles,  which 
taken  together  constitute  indigestion,  or  dyspepsia,  will 
be  postponed  to  the  chapter  on  the  Hygiene  of  Feeding 
(Part  II). 

27.  The  Stimulus  to  the  Secretion  of  the  Gastric  Juice. 
-The  first  requirement  for  the  work  of  the  stomach  is 
the  secretion  of  sufficient  gas- 
tric juice.  Of  late  years  the 
brilliant  researches  of  physi- 
ologists have  shown  that  the 
secretion  of  gastric  juice,  like 
that  of  saliva,  is  excited  by 
impulses  from  the  nervous 
system,  and  that  these  im- 
pulses are  called  forth  in  two 
ways. 

(a)  The  "Psychic"  Secre- 
tion. —  When  agreeable  or  ap- 
petizing food  is  offered  to  an 
animal,  and  especially  when 
such  food  is  taken  into  the 
mouth,  a  secretion  of  gastric 
juice  follows,  which  may  con- 
tinue for  fifteen  minutes  or 
more.  This  secretion  occurs 
when  the  food  has  been  in  the  mouth  only  ten  or  fifteen 
seconds,  and  even  when  it  is  merely  offered  to  a  hungry 
animal  and  not  taken  into  the  mouth  at  all.  Again,  it 


\ 


FIG.  53.  The  curve  of  the  "psychic" 
secretion  of  gastric  juice 

Vertical  lines  represent  half-hour  pe- 
riods after  taking  the  meal ;  hori- 
zontal lines,  relative  amounts  of 
gastric  juice  secreted 


114  THE  HUMAN  MECHANISM 

occurs  only  when  the  animal  is  conscious;  for  if  food  be 
introduced  into  the  stomach  of  a  sleeping  dog,  it  evokes 
only  the  most  scanty  secretion  of  gastric  juice  after  the 
animal  has  awakened.  Moreover,  both  the  amount  and 
the  efficiency  of  the  juice  secreted  vary  directly  with  the 
enjoyment  of  the  meal.  When  meat  is  given  to  a  dog 
which  is  not  hungry,  no  such  abundant  secretion  of  gastric 
juice  occurs  as  during  hunger. 

It  is  clear  that  we  have  here  to  deal  with  a  nervous 
process  more  complicated  than  the  simple  reflex,  and  that 
the  efferent  discharge  to  the  stomach  occurs  as  the  result 
of  nervous  processes  taking  place  in  the  brain  in  connec- 
tion with  the  enjoyment  of  food.  In  other  words,  the  more 
the  food  is  desired  or  enjoyed,  the  more  efficient  will  be 
this  secretion  of  the  gastric  juice. 

It  is  known  that  this  "  psychic  secretion  "  will  continue 
for  several  hours  after  an  ordinary  meal,  increasing  in 
amount  during  the  first  hour  or  more,  and  gradually 
diminishing  from  that  time  onward  (Fig.  53). 

(b)  Reflex  Secretion  from  Stimulation  of  the  Stomach  itself 
by  the  Products  of  Digestion.  —  We  have  just  seen  that  the 
mere  contact  of  most  foods  with  the  walls  of  the  stom- 
ach does  not  call  forth  a  secretion  of  the  gastric  juice. 
This  is  shown  by  the  experiment  of  placing  food  in  the 
stomach  of  a  sleeping  animal ;  such  food,  even  when  appe- 
tizing and  nutritious,  has  been  known  to  remain  in  the 
stomach  for  twelve  or  more  hours  after  the  animal  had 
awakened  without  evoking  a  secretion  of  gastric  juice. 
Yet  it  is  also  known  that  when  digestion  of  food  has  been 
begun  by  the  action  of  the  "  psychic  "  secretion,  the  pep- 
tone formed  from  proteids  arouses  a  second  secretion  which 
increases  in  amount  as  the  first,  or  "psychic,"  secretion 
diminishes.  That  the  peptone  formed  from  proteids  is  the 
true  cause  of  this  secretion  is  proved  by  the  fact  that  the 
introduction  of  peptones  into  the  stomach  of  a  sleeping 


ALIMENTATION  AND  DIGESTION  115 

animal  evokes  this  secretion,  although,  as  already  stated, 
ordinary  food,  which  contains  no  peptones,  does  not.  This 
second  secretion  continues  throughout  the  period  of  gastric 
digestion. 

The  secretion  of  gastric  juice  must  be  kept  up  through- 
out the  entire  period  of  gastric  digestion,  to  make  good  its 
loss  by  the  discharge  of  chyme  into  the  intestine ;  and  it 
would  seem  that  this  great  need  is  met  only  by  the  com- 
bined action  of  the  "  psychic  "  stimulus  and  the  stimulus 
of  peptones  produced  during  the  process  of  digestion. 
Without  the  "  psychic  "  secretion  peptones  are  not  formed 
fast  enough  to  induce  sufficient  subsequent  secretion  ; 
without  the  "peptone"  stimulus  the  psychic"  secretion 
does  not  suffice  to  complete  the  digestion  of  a  hearty 
meal,  a  labor  which  may  require  four  or  five  hours.  It  is 
also  plain  that  proteids  should  form  part  of  large  meals, 
since  it  is  only  from  proteid  that  peptones  can  be  made. 

D.   DIGESTION  AND  ABSORPTION  IN  THE  SMALL  INTES- 
TINE  AND   IN   THE   LARGE   INTESTINE 

Every  few  minutes  during  the  process  of  gastric  diges- 
tion the  pylorus  opens  and  the  stomach  forces  a  few 
cubic  centimeters  of  chyme  into  the  intestine.  Chyme 
consists  of  water  holding  in  solution  certain  products  of 
digestion,  and  in  suspension  relatively  large  quantities  of 
undissolved  matter,  and  has  the  consistency  of  moderately 
thick  pea  soup.  The  suspended  matter  consists,  among  other 
things,  of  small  bundles  of  muscle  fibers  (from  meat),  fat 1 

1  The  fat  of  meat  consists  of  connective  tissue  whose  cells  are  greatly 
swollen  with  drops  of  fat.  In  typical  adipose  tissue  the  connective-tissue 
cell  becomes  one  large  fat  droplet  surrounded  by  the  thin  layer  of  the  cell 
cytoplasm  with  its  nucleus.  These  "fat  cells,"  like  the  muscle  fibers  of 
meat,  are  thus  held  together  by  the  fibers  of  connective  tissue,  and  are 
set  free  when  the  latter  are  digested  and  dissolved  away  by  the  gastric 
juice  (see  Figs.  83-85). 


116 


THE  HUMAN  MECHANISM 


melted  by  the  heat  of  the  body  and  set  free  from  adipose 
tissue  by  the  digestion  of  its  connective  tissue,  bits  of 
coagulated  proteid,  such  as  casein  from  milk  or  the  white 

of  egg,  together  with  other 
proteid  s,  starches,  and  fats 
of  animal  or  vegetable  foods. 
The    work    of    intestinal 
digestion  is  essentially  chem- 
ical in  nature,  and  consists 
largely  in  the  transformation 
of  the  suspended  constitu- 
ents of  chyme  into  soluble 
substances  which  can  be  ab- 
sorbed into  the  blood  and 
furnish  nourishment  for  the 
body;  proteids  are  thus 
changed   into    peptone-like 
bodies,  starches  into  sugars, 
and  fats  probably  into  fatty 
acids  and  soaps.     These 
changes    are    accomplished 
by    digestive    reagents,    or 
juices,  secreted  into  the  in- 
FIG.  EA.  Longitudinal  section  of  the   testine  somewhat  as  the  gas- 
small  intestine  tric  juice  ig  secreted  into  the 
The  submucous  coat  consists  of  connect,    stomach    while  the  mixture 
ive  tissue  and  contains  the   larger  ' 

blood  vessels  from  which  the  mucous    of  food  and  digestive   juice 
and  muscular  coats  are  supplied  with    ig    being    driyen    along   the 

intestine  by  the  action  of  its 

muscular  walls  ;  meanwhile  the  products  formed  are  being 
absorbed  into  the  blood  through  the  lining  membrane  of 
the  intestine. 

28.  The  General  Structure  of  the  Intestine.  —  The  main 
functions  of  the  intestine,  like  those  of  the  stomach,  are 
indicated  in  the  structure  of  two  of  its  coats,  the  muscular 


ALIMENTATION  AND  DIGESTION  117 

coat  and  the  glandular  mucous  membrane.  The  fibers  of 
the  former  are  arranged  in  two  layers,  an  inner  layer  in 
which  they  are  circularly  disposed  around  the  mucous 
membrane  of  the  gut  (see  Fig.  54),  and  a  much  thinner 
outer  layer  in  which  they  run  lengthwise.  The  contraction, 
or  shortening,  of  the  circular  fibers  constricts  the  bore,  or 
lumen,  of  the  tube;  and  it  is  essentially  this  result  that 
the  muscular  work  of  the  intestine  produces. 

In  the  structure  of  the  inner  or  mucous  membrane  two 
points  are  of  importance  to  us.  In  the  first  place  numerous 
simple  tubular  glands  discharge  into  the  intestinal  tube  an 
important  digestive  juice,  the  intestinal  juice ;  in  the  second 
place,  finger-like  processes,  or  villi  (0.5-0.7  mm.  long  by 
0.1  mm.  thick),  arise  from  its  surface  and  project  into  tne 
intestinal  cavity.  These  are  important  organs  of  absorption. 
The  entire  surface  of  the  villi,  the  glands,  and  the  plane 
surface  of  the  intestine  between  these  structures  is  lined 
with  a  continuous  membrane  composed  of  columnar  cells, 
which  separates  blood  vessels  and  lymphatics  in  the  intes- 
tinal wall  from  the  cavity  of  the  intestine  (see  Fig.  55). 

29.  The  Pancreas  and  the  Liver.  —  Besides  the  intestinal 
juice  thus  added  from  the  glands  of  the  mucous  membrane 
to  the  contents  of  the  intestine  as  they  are  passed  along 
its  length  of  twenty  or  twenty-five  feet,  two  other  juices, 
the  pancreatic  juice  and  the  bile,  are  mingled  with  the 
chyme  from  two  large  and  important  glandular  organs 
whose  ducts  open  near  the  stomach.  These  glands  are 
the  pancreas  and  the  liver.  It  is  not  necessary  for  our 
present  purpose  to  describe  the  minute  structure  of  these 
organs  ;  it  is  enough  for  the  student  to  understand  that 
they  are  glands  (p.  29),  which  pour  their  secretions  through 
ducts  into  the  intestine,  very  much  as  the  salivary  glands 
pour  their  secretions  into  the  mouth. 

The  food  in  the  intestine  is  thus  treated  with  three 
principal  secretions,  or  chemical  reagents,  —  the  pancreatic 


118 


THE  HUMAN  MECHANISM 


A 


juice,  the  bile,  and  the  intestinal  juice ;  and  we  may  now 

pass  on  to  a  study  of  these  reagents  and  the  chemical 

reactions  they  cause  in  the  chyme. 

30.  The  Pancreatic  Juice  is  a  strongly  alkaline  liquid, 

and  consequently,  when  mixed  with  the  acid  chyme,  it 

neutralizes  most  if  not  all  the 
hydrochloric  acid  of  the  latter. 
Thus  it  happens  that  while 
the  food  in  the  stomach  is 
strongly  acid,  in  the  intestine 
it  becomes  at  once  either  neu- 
tral or  alkaline.  Under  these 
circumstances  the  pepsin  of 
the  gastric  juice  becomes  in- 
active (see  p.  108),  and  is  soon 
destroyed  by  the  pancreatic 
juice,  so  that  it  plays  no  fur- 
ther r61e  in  proteid  digestion. 
This  is  henceforward  carried 
on  by  an  enzyme  of  the  pan- 
creatic juice,  trypsin,  which 
acts  most  vigorously  in  a 

FIG.  55.  Longitudinal  section  of     neutral    or    slightly    alkaline 


the  tip  of  a  villus 


medium.    It  forms  from  the 


Showing:  the  columnar  lining  cells  »  . ,       ,,       ,    . , 

B,  through  which  the  products  of    proteids  of  the  f  ood  the  same 
digestion  must  pass  on  their  way     general  class  of  peptone-like 

to  the  blood  vessels  and  lymphat-  •,     ,  ^          11,1 

ics.  The  connective  tissue  he-    substances  produced  by  the 

tween  the  columnar  cells  and  the      action    of    the    gastric    juice, 
vessels  is  indicated  diagrammati-      T  jj-,- 

caiiy  and  without  showing  its     In    addition    to   trypsin    the 
structure.   A,  cell  which  manu-    pancreatic   juice    contains   at 

factures  mucus  ;    C,  capillaries  ;      i  ,  r  ,  i 

D,  lacteal  or  lymphatic  least    three     °ther     enzymes  ; 

one  of   which    seems    to    be 

closely  related,  if  not  identical  with,  the  rennin  of  the 
stomach,  although  the  exact  part  it  plays  in  digestion  is 
not  yet  clear.  The  other  two  enzymes  are  much  more 


ALIMENTATION  AND  DIGESTION  119 

important.  One  of  them,  amylopsin,  is  practically  identical 
with  the  ptyalin  of  the  saliva,  and  changes  starch  into 
sugar,  much  as  happens  in  salivary  digestion.  The  other 
enzyme,  lipase  or  steapsin,  acts  upon  fats,  changing  them 
into  fatty  acids  and  glycerin.  We  cannot  go  into  the 
details  of  the  somewhat  complicated  digestion  of  fats. 
The  change,  like  that  of  proteids  into  peptones  and  of 
starches  into  sugar,  involves  the  formation  of  a  smaller 
molecule,  either  of  fatty  acids,  or  soaps,  or  both,  and  it 
is  probably  in  these  forms  that  fats  are  received  from  the 
intestine  by  the  villi. 

The  pancreatic  juice  thus  contains  a  special  enzyme  for 
each  of  the  three  great  classes  of  nutrients,  —  proteids,  fats, 
and  carbohydrates,  —  and  thoroughly  completes  their  diges- 
tion, after  they  have  undergone  the  preparatory  processes 
effected  by  cooking,  mastication,  and  gastric  digestion. 
Pancreatic  juice  is  by  far  the  most  important  of  the  digest- 
ive juices  in  producing  the  chemical  changes  of  digestion. 
In  this  respect,  also,  we  may  say  it  is  of  primary  impor- 
tance in  the  work  of  intestinal  digestion,  the  other  two 
juices,  the  bile  and  the  intestinal  juice,  acting  as  aids  in 
its  work. 

31.  The  Bile  contains  no  enzymes  of  importance  in 
digestion.  It  is  in  fact  partly  an  excretion,  some  of  its 
constituents  being  waste  products  which  are  poured  into 
the  intestine  only  to  be  ultimately  discharged  from  the 
rectum.  Other  constituents  of  the  bile  play  an  important 
rdle  in  the  digestion  and  absorption  of  fats,  as  is  shown 
by  the  fact  that,  if  bile  be  prevented  from  entering  the  in- 
testine, from  forty  to  sixty  per  cent  of  the  fat  eaten  fails  of 
absorption  and  is  discharged  with  the  f  eces.  It  is  probable 
that  this  is  because  certain  soaps  formed  in  pancreatic 
digestion  are  not  soluble  unless  bile  is  present.  When 
these  soaps  are  not  dissolved  they  are  not  only  themselves 
not  absorbed,  but,  by  being  precipitated  and  adhering  to 


120  THE  HUMAN  MECHANISM 

other  still  undigested  food,  prevent  the  ready  access  of  the 
enzymes,  and  so  greatly  delay  digestion. 

32.  The  Intestinal  Juice,  like  the  bile,  contains  no  enzymes 
of  primary  importance,  although  it  contains  some  which 
play  a  subordinate  rdle.    It  is,  however,  characterized  by 
its  large  content  of  alkaline  salts,  especially  sodium  car- 
bonate.    This  is  of  importance,  since  two  processes  not 
yet  emphasized  but  constantly  occurring  in  the  intestine 
produce  acid ;  these  are  (a)  the  splitting  of  the  fats  into 
fatty  acids  and  glycerin,  and  (b)  the  bacterial  decomposi- 
tions of  carbohydrates  and  (to  some  extent)  of  proteids. 
The  sodium  carbonate  of  the  intestinal  juice  which,  it  will 
be  remembered,  is  being  secreted  along  the  entire  length 
of  the  intestine,  neutralizes  these  acids  and  so  maintains 
the  reaction  of  the  contents  at  an  approximately  neutral 
point.    This  reaction  is  most  favorable  for  the  action  of 
the  enzymes  present. 

33.  Action  of  the  Muscular  Coat  of  the  Intestine.  — Few 
points  in  the  structure  and  functions  of  the  intestine  are 
more  striking  than  the  provisions   made   for  promoting 
mechanically  by  muscular  movements  the  effective  mixture 
of  the  food  received  from  the  stomach  with  the  digestive 
juices  and  abundant  contact  of  the  digested  foods  with  ab- 
sorbing surfaces.    Consider,  to  begin  with,  the  structure  of 
the  intestine.    The  average  length  of  the  small  intestine 
is  about  twenty  feet;   its  average  circumference  is  some 
three  inches ;  this  of  itself  would  give  an  absorbing  sur- 
face of  some  five  square  feet.    But  this  surface  is  increased 
fivefold  or  more  by  the  finger-like  villi  and  by  folds  of 
the  mucous  membrane ;  so  that  we  may  estimate  the  total 
area  of  the  absorbing  surface  as  probably  not  less  than 
twenty-five  square  feet,  an  area  almost  twice  as  great  as 
that  of  the  skin.    But  this  is  not  the  whole  story.    If  the 
digesting  mixture  in  the  intestine  were  kept  in  constant 
motion,  it  is  clear  that  contact  of  all  parts  of  it  with  the 


ALIMENTATION  AND  DIGESTION 


121 


villi  would  be  greatly  facilitated;  and  such  a  motion,  or 
agitation,  is  in  fact  maintained  by  contraction  of  the  mus- 
cles of  the  intestinal  walls  and  the  consequent  movement 
of  the  contents. 

34.  Divisive   or   Segmenting  Movements.  —  The   move- 
ments of  the  intestine,  like  those  of  the  stomach,  are  best 


FIG.  56.  The  divisive  or  segmenting  movements  of  the  small  intestine 

A,  surface  view  of  a  portion  of  the  intestine,  showing  six  constrictions  which 
divide  the  contents  into  five  segments,  as  shown  in  B ;  as  these  constric- 
tions pass  away  new  ones  come  in  between  them  and  divide  each  segment 
of  the  contents  into  two,  the  adjoining  halves  of  neighboring  segments 
fusing  to  make  the  new  segments  shown  in  C.  Repetition  of  this  process 
results  in  the  condition  shown  in  D 

studied  by  means  of  the  Rontgen  rays.  These  studies 
have  shown  that  the  food  is  not  distributed  continuously 
along  the  entire  length  of  the  intestine,  but  is  divided  into 
a  number  of  separate  portions  which  lie  in  different  loops 
of  the  intestine.  This  is  partly  due  to  the  intermittent 
character  of  the  discharge  of  the  chyme  from  the  stom- 
ach. A  certain  number,  sometimes  all,  of  these  masses  of 
food  will  be  seen  to  undergo  division  into  small  segments, 


122  THE  HUMAN  MECHANISM 

obviously  produced  by  a  series  of  constrictions  of  the 
walls,  as  shown  in  Fig.  56.  The  next  moment  these  are  re- 
placed by  a  second  series  of  constrictions  between  the  first. 
Each  segment  is  thus  divided  into  two,  and  the  neighbor- 
ing halves  of  these  segments  fuse.  The  next  moment  the 
second  series  of  constrictions  is  replaced  by  the  first,  and 
this  process  continues  at  times  for  many  minutes  with  no 
change  in  the  general  position  of  the  food  mass.  These 
divisive,  or  segmenting,  movements  occur  from  twenty  to 
thirty  times  a  minute,  and  it  has  been  estimated  "  that  a 
slender  string  of  food  may  commonly  undergo  division  into 
small  particles  more  than  a  thousand  times  while  scarcely 
changing  its  position  in  the  intestine." 

35.  Peristalsis.  —  Every  now  and  then  a  ring  of  con- 
striction, instead  of  being  confined  to  one  place,  moves 
onward,  pushing  before  it  the  contents  of  the  tube.  A 
contraction  of  this  kind  is  called  peristaltic.  The  effect 
produced  is  much  the  same  as  when  the  contents  of  a 
rubber  tube  are  emptied  by  squeezing  it  along  between 
the  thumb  and  finger ;  and  it  is  by  this  means  that 
the  contents  of  the  small  intestine  are  from  time  to 
time  moved  onwards  from  the  stomach  toward  the  large 
intestine. 

Thus  each  consignment  of  chyme  delivered  from  the 
stomach  immediately  receives  its  share  of  pancreatic  juice 
and  of  bile,  and  the  final  transformation  of  the  digestible 
foods  takes  place  as  the  whole  is  driven  from  time  to 
time  along  the  intestine  by  peristaltic  contractions,  the 
efficiency  of  the  contact  of  the  food  with  the  digestive 
juices,  as  well  as  its  exposure  to  the  absorbing  surfaces, 
being  greatly  enhanced  by  the  agitation  produced  by  the 
movements  of  constrictive  division  carried  out  by  the  cir- 
cular muscles  between  periods  of  peristaltic  activity.  The 
efficiency  of  digestion  and  absorption  probably  depends  as 
much  on  the  movements  carried  out  by  the  muscular  coat  as 


ALIMENTATION  AND  DIGESTION 


123 


upon  the  chemical  processes  effected  by  the  enzymes  and  the 
ether  constituents  of  the  digestive  juices. 

So  far  as  is  known,  the  stimulus  to  these  movements  is 
afforded  by  the  presence  of  mate- 
rial within  the  intestine,  and  es- 
pecially by  the  contact  of  solid 
particles  with  the  walls;  but  we 
cannot  enter  into  detail  on  this 
point  further  than  to  say  that  ner- 
vous factors  are  known  to  play  an 
important  part. 

36.  Absorption  is  the  name  given 
to  the  passage  of  digested  food 
materials  from  the  cavity  of  the 
intestine  into  the  blood.  The  word 
itself  perhaps  suggests  that  the 
products  of  digestion  are  received 
into  the  blood  without  change,  as 
a  .sponge  might  absorb  a  solution 
of  peptone,  sugar,  fatty  acids, 
soaps,  and  inorganic  salts.  Such, 
however,  is  by  no  means  the  case,  FlG.  57>  The  intestinal  struc- 
and  the  actual  physical  and  chem-  tures  concerned  in  ab- 
ical  processes  of  absorption  are 
extremely  complicated,  —  far  too 
complicated  to  be  discussed  here. 
Suffice  it  to  say  that  the  intestine 
is  not  lined  by  a  dead  membrane 
but  by  living  cells,  and  through 
these  guardian  cells  the  products 
of  digestion  must  pass  to  enter  the 
blood  (see  figure  of  a  villus,  p.  11 8). 
It  is  known  that  the  peptone-like  bodies  are  not  allowed 
to  enter  the  blood  (except  in  mere  traces),  and  it  is  gen- 
erally believed  that  they  are  again  built  up  into  certain 


immediately  under  the  lin- 
ing membrane ;  in  the  other 
villus,the  central  lymphatic 
or  lacteal.  Observe  that  the 
products  of  digestion  must 
first  be  exposed  to  absorp- 
tion by  the  blood  vessels 
before  they  can  enter  the 
lacteal 


124  THE  HUMAN  MECHANISM 

definite  proteids  which  can  nourish  the  tissues.  Again, 
the  fats,  though  they  enter  the  lining  membrane  as  soaps 
or  fatty  acids,  appear  in  the  blood  largely  as  fats.  The 
grape  sugar,  on  the  other  hand,  formed  in  the  digestion  of 
starch,  probably  passes  into  the  blood  unchanged. 

The  present  state  of  our  knowledge  does  not  justify 
further  discussion  of  the  full  significance  of  these  changes. 
But  what  has  been  said  is  enough  to  show  that  the  chemical 
changes  of  digestion  do  not  end  with  those  produced  by 
enzymes  or  other  agents  in  the  cavity  of  the  intestine, 
but  that  these  are  succeeded  by  other  important  changes 
effected  during  absorption.  The  object  of  the  whole  process 
of  alimentation,  digestion,  and  absorption  would  seem  to 
be  that  of  supplying  food  to  the  muscle  fiber,  the  gland 
cell,  the  nerve  cell,  etc.,  through  the  blood  as  an  internal 
medium,  or  middleman,  in  that  form  which  is  best  fitted 
for  the  use  of  the  tissues.  No  matter  how  varied  the  forms 
of  proteid  (meats  of  various  kinds,  beans,  peas,  white  of 
egg,  curd  of  milk,  etc.),  all  would  seem  to  enter  the  blood, 
after  being  worked  over  by  the  processes  of  digestion  and 
absorption,  in  the  form  of  a  few  definite  compounds,  pos- 
sibly of  proteid  nature,  which  are  utilized  by  the  tissues 
with  comparative  ease ;  and  something  similar  is  true  of 
fats  and  carbohydrates.  It  is  very  instructive  to  find  that 
whereas  grape  sugar  and  fruit  sugar,  when  introduced 
directly  into  the  blood,  nourish  the  tissues  of  the  body  as 
well  as  when  they  are  received  from  thte  alimentary  canal, 
the  closely  related  cane  sugar  when  so  injected  is  use- 
less ;  taken  as  a  food  into  the  alimentary  canal,  the  latter 
is  there  changed  into  grape  sugar  and  fruit  sugar,  and 
absorbed  in  these  forms,  it  nourishes  the  body.  Simi- 
larly, the  raw  or  unboiled  white  of  egg  is  useless  as  a 
food  unless  it  first  be  changed  into  other  proteids  by  the 
processes  of  digestion.  Indeed,  it  is  in  general  correct  to 
look  upon  the  chemical  processes  of  internal  digestion  as 


ALIMENTATION  AND  DIGESTION 


125 


merely  initiating  a  long  series  of  successive  chemical 
changes,  which,  beginning  in  the  intestine,  are  continued 
in  the  process  of  absorption  and  only  completed  in  the 


FIG.  58.  The  paths  by  which  the  products  of  digestion  enter  the  general 

circulation 

Those  which  are  absorbed  by  the  blood  vessels  O  of  the  intestine  pass  by  the 
portal  vein  P.  V.  to  the  liver  before  they  can  enter  the  right  auricle 
R.A.  through  the  hepatic  vein  H.  V.  and  the  inferior  vena  cava  I.V.G. 
Those  products  which  are  absorbed  by  the  lacteals  pass  directly  to  the 
superior  vena  cava  S.  V.  C.  through  the  thoracic  duct 

tissues  themselves.    The  last  stage  of  these  changes  we 
have  yet  to  study  in  the  chapters  on  nutrition. 

37.  Microbic  Life  in  the  Intestine.  —  Occurring  simultane- 
ously with  the  chemical  changes  produced  by  the  digestive 
juices  are  others  produced  by  microbes  (Part  II),  which 
are  always  found  in  the  intestine  in  large  quantities.  The 
acidity  of  the  gastric  juice  keeps  down  the  numbers  of 
these  germs  in  the  stomach,  and  under  healthy  conditions 
greatly  limits  their  activity  in  that  organ.  We  have  seen, 
however,  that  some  portions  of  the  contents  of  the  stomach 
are  not  acid  in  reaction  during  certain  periods  of  digestion ; 
and  it  not  infrequently  happens  for  this  reason  that  un- 
healthy living,  and  especially  improper  feeding,  may  result 
in  serious  gastric  indigestion  with  excessive  bacterial  decom- 
position of  the  food.  The  production  of  gas,  leading  to 
flatulence  or  belching,  is  one  of  the  most  familiar  results 
of  such  bacterial  action. 


126  THE  HUMAN  MECHANISM 

In  the  intestine  the  approximately  neutral  or  even  alka- 
line reaction  is  much  more  favorable  to  bacterial  life  and 
growth,  and  we  accordingly  find  that  the  number  of  microbes 
gradually  increases  from  the  stomach  to  the  large  intestine. 
It  is  not  the  microbe  itself,  however,  which  is  of  impor- 
tance to  the  organism  as  a  whole,  but  the  substances  which 
it  produces  from  the  foods.  Most  of  these  are  either  harm- 
less themselves  or  else  are  readily  changed  into  harmless 
substances,  either  before  or  soon  after  entering  the  blood ; 
it  has  even  been  suggested  that  some  are  useful  and  that 
microbic  life  is  a  valuable  aid  to  digestion,  —  an  idea  that 
has  no  substantial  basis;  others  are  poisons,  but  are  nor- 
mally present  in  such  minute  quantities  as  to  be  entirely 
negligible ;  more  rarely  they  are  produced  in  large  quanti- 
ties and  may  cause  various  ill  effects  either  locally  or  upon 
the  body  as  a  whole. 

The  production  -of  undue  quantities  of  such  harmful 
substances,  most  of  which  are  derived  from  proteids,  is 
chiefly  dependent  upon  the  food  supply  of  the  bacteria. 
This  is  normally  kept  low  by  the  speedy  and  efficient 
removal  of  the  peptones.  Native 1  proteids  are  acted  on  com- 
paratively slowly  by  bacteria,  and  in  any  case  must  first 
be  changed  into  peptones  before  they  can  be  further  broken 
down  into  harmful  bodies.  If,  however,  the  processes  of 
absorption  quickly  and  efficiently  remove  the  peptones 
formed  both  by  the  bacteria  and  by  the  enzymes,  subse- 
quent harmful  decomposition  of  the  food  is  prevented,  for 
there  are  normally  no  bacteria  in  the  blood.  It  is,  there- 
fore, of  great  importance  to  maintain  the  efficiency  of 
absorption.  This  can  be  done  in  general  only  by  leading 
a  normal  life,  —  by  taking  sufficient  muscular  exercise,  by 

1  A  "  native  "  proteid  is  a  proteid  in  its  natural  form,  or  as  it  occurs 
in  nature  before  being  changed  by  digestion  or  other  chemical  action. 
The  proteids  in  food  are  largely  native  proteids,  or  else,  what  amounts 
to  the  same  thing,  as  far  as  the  action  of  bacteria  is  concerned,  native 
proteids  coagulated  by  heat. 


ALIMENTATION  AND  DIGESTION 


127 


proper  habits  of  sleep  and  rest,  by  proper  feeding,  and  so 
on.  The  hygienic  conduct  of  life  tends  to  maintain  all 
functions  of  the  body  in  proper  working  condition,  those 
of  the  digestive  organs  included;  and  nothing  else  can  be 
depended  on,  in  the  long  run,  to  do  this.  To  this  subject 
we  shall  return  in  the  chapters  on  hygiene,  when  dealing 
directly  with  the  personal  conduct  of  life. 

We  have  thus  far  been  dealing  only  with  those  microbes 
commonly  found  in  the  intestine.  At  times  foreign  mi- 
crobes find  entrance,  and  these  may  be  the  cause  of  such 
diseases  as  typhoid  fever,  dysentery,  cholera,  etc.  The 
action  of  these  occasional  intruders  will  be  more  fully 
dealt  with  in  Part  II. 

38.  Digestion  in  the  Large  Intestine.  —  The  large  intes- 
tine contains  no  villi,  and  its  glands  secrete  a  somewhat 
different  intestinal  juice  from 
that  of  the  small  intestine.  It 
is  a  juice  characterized  by  a 
larger  percentage  of  the  vis- 
cous substance,  mucin,  familiar 
in  the  nasal  discharge  during  a 
cold,  and  in  the  thick,  glairy 
saliva  sometimes  secreted.  This 
juice  serves  chiefly  to  lubricate 
the  walls  of  the  intestine  and 
to  prevent  harmful  friction 
with  the  more  solid  contents. 

In  the  small  intestine  the 
amount  of  water  added  by  se- 
cretion balances  that  absorbed,  FIG.  59.  Longitudinal  section  of 
so  that  the  consistency  of  the 
contents  undergoes  but  little 
change  from  the  stomach  to  the  beginning  of  the  large  in- 
testine. This  consistency,  it  will  be  remembered,  was  (ap- 
proximately) that  of  moderately  thick  pea  soup.  During 


the  large  intestine 
Note  the  absence  of  villi 


128  THE  HUMAN  MECHANISM 

the  passage  through  the  small  intestine  the  digested  por- 
tions of  the  food  are  being  removed  by  absorption,  while 
the  indigestible  elements  are  left  behind.  Among  the  indi- 
gestible elements  of  food  are  certain  connective  tissues  of 
the  animal  foods,  but  especially  the  cellulose  (p.  94),  which 
forms  the  cell  wall  of  plant  tissues.  The  large  intestine 
receives  from  the  small  this  indigestible  material,  together 
with  a  certain  variable  but  usually  comparatively  small 
proportion  of  the  proteids,  fats,  and  carbohydrates  which 
have  thus  far  escaped  digestion ;  in  addition  there  are  cer- 
tain constituents  of  the  digestive  juices  which  are  not 
absorbed,  and  some  (e.g.  certain  constituents  of  the  bile) 
which  are  distinctly  excretory  products. 

The  digestion  of  proteids,  fats,  and  carbohydrates  may 
to  some  extent  continue  in  the  large  intestine,  and  the 
products  of  digestion  be  absorbed.  This  process,  however, 
tends  to  be  checked,  owing  to  the  fact  that  here  the 
processes  of  secretion  fall  behind  those  of  absorption, 
so  that  the  contents  of  the  intestine  gradually  become 
more  and  more  solid,  being  finally  collected  as  the  feces 
in  the  lower  colon  and  rectum.  As  the  intestinal  contents 
become  moje  solid,  the  peptones  and  other  digestive  prod- 
ucts are  less  efficiently  removed  while  bacterial  action 
greatly  increases,  giving  to  the  feces  their  characteristic 
odors.  And  while  the  substances  which  are  responsible 
for  the  offensive  odors  of  the  feces  may  themselves  be 
harmless  to  the  body,  it  must  be  remembered  that  such 
odors  are  an  indication  of  bacterial  action  capable  of  pro- 
ducing other  substances,  which  may  be,  and  very  often  are, 
the  cause  of  headaches,  general  malaise,  etc.,  or  of  even 
more  serious  troubles. 

39.  The  Elimination  of  Intestinal  Waste.  —  Those  who 
are  "  blessed  with  a  good  digestion "  sometimes  find  it 
hard  to  realize  that  the  preparation  of  food  for  absorp- 
tion through  the  delicate  membranes  lining  the  alimentary 


ALIMENTATION  AND  DIGESTION  129 

canal  is  a  difficult  and  complex  process,  requiring  much 
delicate  physical  and  physiological  apparatus  and  involv- 
ing various  and  important  chemical  reactions.  Even  when 
they  realize  this,  they  rarely  appreciate  the  indispensable 
cooperation  and  fine  adjustment  of  the  several  parts  and 
processes  concerned.  It  is  just  here,  however,  that  a  clear 
understanding  is  important,  for  without  this  it  is  not  easy 
to  see  how  disorders  of  digestion  arise. 

Let  us  then  remember  that  the  efficient  handling  of  the 
food  in  the  stomach  is  aided  by  the  preparatory  crushing 
it  receives  in  the  process  of  mastication  ;  that  in  the  stom- 
ach an  adequate  and  efficient  secretion  of  gastric  juice 
must  take  place,  and  that  this  begins  as  the  result  of  nerv- 
ous events  connected  with  our  enjoyment  of  the  food 
when  eaten  ;  that  the  continued  secretion  of  gastric  juice 
is  secured,  in  turn,  by  stimulation  of  the  mucous  membrane 
of  the  stomach  by  the  peptones  which  the  "  psychic  "  secre- 
tion has  formed  from  the  proteids  of  the  food  ;  and  finally 
that  the  chemical  action  of  the  gastric  juice  is  aided  by  the 
peculiar  contractions  of  the  muscular  coat  of  the  stomach. 
All  these  agencies  working  together  deliver  the  food  to  the 
intestine  in  a  finely  divided  state,  well  adapted  and  indeed 
absolutely  necessary  to  secure  the  proper  contact  of  the 
food  with  the  pancreatic  juice,  the  bile,  and  the  intestinal 
juice. 

The  flow  of  pancreatic  juice,  in  turn,  is  partly  the  result 
of  the  action  of  the  hydrochloric  acid  of  the  chyme  on  the 
walls  of  the  intestine,  while  the  efficiency  of  the  action  of 
the  pancreatic  enzymes  depends  upon  the  simultaneous 
action  of  the  bile  and  the  intestinal  juice;  lastly,  the 
chemical  action  of  these  juices,  as  well  as  the  final  act  of 
absorption,  requires  the  cooperation  of  the  muscular  coat. 
Healthy  conditions  with  respect  to  bacterial  action  simi- 
larly depend  upon  all  else  occurring  as  it  should.  Diges- 
tion, in  short,  is  a  chain  of  events,  each  depending  upon 


130  THE  HUMAN  MECHANISM 

those  which  have  gone  before,  and  to  a  large  extent  upon 
those  which  are  taking  place  at  the  same  time. 

Keeping  these  facts  in  mind,  it  is  easy  to  appreciate  the 
possibility  of  diarrhea  or  constipation,  the  latter  consist- 
ing in  the  retention  of  wastes,  the  poisonous  constituents 
of  which  may  be  absorbed  into  the  body  and  cause  dis- 
comfort, headaches,  and  malaise.  When  all  the  digestive 
processes  work  together  properly  there  should  be  a  per- 
fectly natural  and  regular  evacuation  of  the  bowels.  The 
frequency  of  such  evacuation  varies  somewhat  and  is 
largely  a  matter  of  habit ;  with  some  people  it  is  twice  a 
day,  with  others  once  every  other  day,  but  with  the  vast 
majority  it  is  normally  once  every  day  and?  at  about  the  same 
time.  Where  this  is  not  the  case  there  is  reason  to  believe 
that  some  part  of  the  work  of  digestion  is  not  being  prop- 
erly performed.  The  trouble  is  not  ordinarily  in  the  mech- 
anism governing  the  actual  discharge  of  the  feces  from  the 
rectum,  but  in  a  derangement  somewhere  else ;  it  may  be 
entirely  the  fault  of  the  mechanism  of  peristalsis,  or  it  may 
be  due  to  imperfect  secretion.  In  all  cases  it  means  that 
something  is  wrong,  and  the  remedy  should  be  sought  not 
in  drugs  or  pills  but  in  search  for  and  removal  of  the 
cause.  A  moment's  consideration  will  show  the  reasonable- 
ness of  this  position.  If  a  watch  loses  time  because  it  needs 
cleaning,  we  do  not  seek  a  remedy  in  drugs,  but  in  its 
cleaning,  better  adjustment,  and  good  care ;  and  the  rem- 
edy for  diarrhea  or  constipation  should  in  all  cases  be 
sought  for  in  the  better  conduct  of  life.  Is  enough  mus- 
cular exercise  being  taken  ?  Is  the  diet  properly  chosen  ? 
Are  we  drinking  enough  water  ?  Especially,  is  the  food  of 
sufficient  bulk,  and  does  it  contain  enough  laxative  mate- 
rial (such  as  fruit)  ?  And,  above  all,  are  we  getting  enough 
sleep  ?  Are  we  overworking,  or  do  we  work  too  long  at  a 
time  without  resting?  Is  our  clothing  warm  enough,  or 
are  we  overclad?  Such  are  the  questions  which  should  be 


ALIMENTATION  AND  DIGESTION  131 

seriously  asked.  The  student  of  personal  hygiene  cannot 
lay  to  heart  too  seriously  the  truth  that  the  man  who  goes 
from  day  to  day,  from  week  to  week,  from  year  to  year, 
neglecting  the  warnings  of  diarrhea  or  constipation,  only 
reaps  the  harvest  of  his  folly  when  in  later  years  he  suffers 
loss  of  health  and  at  times  bodily  discomfort ;  and  it  is  noth- 
ing short  of  impiety  to  marvel  under  such  circumstances  at 
the  "  mysterious  "  ways  of  a  Providence  which  so  "  afflicts  " 
his  creatures.  It  is  no  exaggeration  to  say  that  the  regular 
discharge  of  the  wastes  is  quite  as  important  as  the  regular 
feeding  of  the  body,  and  that  no  less  pains  should  be  taken 
to  form  good  habits  in  the  one  case  than  in  the  other. 
Many  of  the  headaches,  many  of  the  bad  feelings,  and 
many  of  the  bad  tempers  of  the  world  are  due  to  neglect 
of  this  simple  fact.  No  city,  however  well  fed  or  beautiful, 
the  drains  of  which  are  choked  with  filth,  can  long  remain 
either  wholesome  or  attractive,  —  and  the  human  body  is 
essentially  a  city  populous  with  living  cells. 


CHAPTER  IX 

THE  CIRCULATION  OF  THE  BLOOD 
A.  BLOOD  AND  LYMPH 

1.  The  Blood  as  a  Common  Carrier.  —  In  previous  chap- 
ters some  of  the  more  general  features  of  the  circulation 
have  already  been  touched  upon.  In  studying  the  parts  of 
the  body  the  student  has  become  somewhat  acquainted 
with  the  heart,  the  arteries,  and  the  veins ;  in  considering 
the  typical  structure  of  the  organs  (Chapter  III)  he  has 
seen  how  the  arteries  are  connected  with  the  veins  by  a 
system  of  communicating  tubes,  the  capillaries,  through 
the  thin  walls  of  which  interchange  takes  place  between 
the  lymph  and  the  blood  ;  and  in  studying  the  interdepend- 
ence and  cooperation  of  the  cells  and  organs  (Chapter  VI) 
he  has  learned  how  the  blood  leaving  each  organ  returns 
to  the  heart,  there  to  be  mixed  with  that  coming  from  all 
other  organs  and  thence  pumped  first  to  the  lungs  and 
then  to  the  rest  of  the  body.  The  need  of  a  circulation  is 
obvious ;  for  the  food  received  from  the  alimentary  canal 
and  the  oxygen  received  from  the  lungs  must  somehow  be 
carried  to  the  muscle  fibers,  the  nerve  cells,  the  gland 
cells  ;  the  cellular  wastes  must  be  taken  away  to  the 
organs  of  excretion,  and  the  internal  secretions  of  the 
body  must  be  transported  from  the  organs  in  which  they 
are  made  to  those  in  which  they  are  to  be  used.  In  other 
words,  it  is  a  necessary  corollary  to  the  fact  that  no  cell 
or  organ  "  liveth  unto  itself  "  that  there  should  be  some 
common  carrier  of  matter  and  of  energy  from  one  organ 
to  another.  Such  a  common  carrier  is  the  blood.  The 

132 


THE  CIRCULATION  OF  THE  BLOOD         133 


analogy  of  the  blood  system  of  the  body  with  the  railway 
system  of  a  country  is  instructive.  As  different  persons 
and  different  communities  in  any  country  make  different 
products  and  have  dif- 

"V 


ferent  needs,  it  becomes 
more  and  more  neces- 
sary that  the  means  of 
communication  between 
them  be  extensive  and 
efficient.  Hence  the  re- 
markable growth  of  the 
railroads,  or  "  common 
carriers,"  of  any  coun- 


try in  which  industrial 
development  produces 
increasing  division  of 
labor. 

The  blood,  which  is 
thus  the  common  carrier 
first  between  the  various 
organs,  and  second  be- 
tween each  organ  and 
the  outer  environment, 
is  the  net  product  of  the 
united  work  of  all  the 
organs  ;  from  the  alimen- 
tary canal  it  receives 
water  and  the  products 
of  digestion ;  from  the 


FIG.  60.  Structure  of  a  drop  of  blood  as 
seen  under  the  microscope 

Above  are  shown  nine  red  corpuscles,  highly 
magnified;  below,  less  highly  magnified, 
the  appearance  of  the  blood  soon  after 
being  drawn.  Two  white  corpuscles  are 
shown,  and  the  red  corpuscles  stick  to- 
gether, forming  "  rouleaux."  Size  of  red 
corpuscle,  7.7 >  wide,  2-4 /*  thick.  Diam- 
eter of  white  corpuscle,  5-10  /".  Number 
of  red  corpuscles,  4,500,000-5,000,000  per 
cubic  millimeter.  Number  of  white  cor- 
puscles, 4500-13,000  per  cubic  millimeter, 
according  to  the  state  of  digestion,  etc. 
Surface  area  of  all  the  red  corpuscles 
of  the  blood,  3000  square  meters  (30,000 
square  feet  or  approximately  four  times 
the  size  of  a  baseball  diamond.  (1>  or 
micron  —  0.001  millimeter) 


lungs  it  receives  oxygen ; 
each  organ  contributes  its  share  of  waste  products  or  of  in- 
ternal secretion,  while  some  influence  the  composition  of  the 
blood  by  removing  from  it  certain  things  that  it  contains. 
2.  The  Microscopic  Structure  of  the  Blood.  —  Examined 
under  the  microscope   the  blood  is  seen  to  consist  of  a 


134 


THE  HUMAN  MECHANISM 


liquid  portion,  the  plasma,  crowded  with  small  solid  bodies, 
the  corpuscles.  These  are  of  two  kinds :  the  red  corpuscles, 
—  biconcave  disks  containing  a  pigment,  hemoglobin,  which 
gives  the  red  color  to  the  blood ;  and  the  white  corpuscles, 
which  are  colorless,  nucleated  cells. 

Important  data  on  the  number,  size,  and  surface  area  of 
the  corpuscles  will  be  found  in  connection  with  Fig.  60. 

3.  The  White  Blood  Corpuscles.  —  The  white  blood  cor- 
puscles really  comprise  several  different  kinds  of  cells,  hav- 
ing different  functions, 
the  study  and  explana- 
tion of  which  belong  to 
advanced  rather  than  ele- 
mentary physiology.  It 
is  enough  for  our  pur- 
pose to  state  that  these 
cells  are  not  confined  to 
the  blood,  but  work  their 
way  out  of  the  blood  ves- 
sels between  the  cells  of 

3  the   capillary  walls  and 

FIG.  61.  Amoeboid  movement  of  a  white    are    often   found   in    the 
corpuscle  lymph  spaces  of  the  tis- 

Showing  four  consecutive  positions  among     gues    ag   w(mderinq   cells. 
a  group  of  red  corpuscles  „..  . 

The  latter  term  refers  to 

their  movement  from  place  to  place.  The  cytoplasm  of  the 
white  corpuscle  is  a  thick,  viscous  fluid  without  constant 
or  definite  form.  In  locomotion  the  cytoplasm  flows  slowly 
from  some  part  of  the  surface  in  the  direction  of  motion, 
forming  what  is  known  as  a  pseudopodium  (from  the  Greek  ; 
meaning  a  false  foot),  as  shown  in  Fig.  61 ;  the  rest  of  the 
body  of  the  corpuscle  then  flows  into  the  pseudopodium. 
By  the  continuation  of  this  process  the  white  corpuscles 
make  their  way  through  the  spaces  of  the  connective  tissue. 
Locomotion  by  means  of  pseudopodia  is  frequently  spoken 


THE  CIRCULATION  OF  THE  BLOOD         135 

of  as  amceboid,  from  the  amoeba,  a  unicellular  animal  which 
moves  in  the  same  manner. 

4.  The  Red  Blood  Corpuscles.  —  The  red  corpuscles  are 
pigmented,  biconcave  disks,  with  no  nucleus  ;  they  are  nor- 
mally confined  to  the  blood  vessels  and  are  carried  around 
passively  in  the  blood  current  without  active  movements 
of  their  own.    The  main  function  of  these  corpuscles  is 
to  carry  oxygen  from  the  lungs  to  the  tissues,  a  function 
which  will  be  further  studied  in  connection  with  respira- 
tion.   They  contain  a  pigment,  hemoglobin,  which  gives  to 
the  blood  its  red  color  and  carries  the  oxygen. 

5.  The  Blood  Plasma  is  an  exceedingly  complex  fluid 
whose   general    composition    is    represented    as    follows : 
water,  90  parts;  solids,  10  parts,  —  proteids,  8  parts;  inor- 
ganic salts,  1  part;  extractives,  1  part. 

Under  the  extractives  are  included  a  very  large  num- 
ber of  substances  which,  though  present  in  small  quantities, 
are  interesting  to  the  physiologist  because  they  are  largely 
products  of  the  chemical  activities  of  the  body,  and  as  such 
give  information  about  the  nature  of  the  chemical  changes 
occurring  in  the  organs. 

Finally,  it  should  be  remembered  that  the  cells  of  the 
body  generally  are  bathed  with  lymph,  not  with  blood; 
in  other  words,  that  the  lymph  and  not  the  blood  is  the 
immediate  environment  of  the  cells.  Lymph  is  sometimes 
described  as  blood  minus  its  red  corpuscles,  but  this  state- 
ment, though  convenient,  is  not  strictly  correct,  since  the 
amount  of  waste  products  in  lymph  must  be  greater  than 
in  blood,  while  the  amount  of  food  material  must  be 
less  (see  Chapter  IV).  Much  as  the  blood  is  a  product  of 
the  united  chemical  activity  of  all  the  organs  of  the  body, 
so  the  lymph  of  each  organ  is  derived  from  the  cells  of 
that  organ  and  from  the  blood  flowing  through  it.  Lymph 
thus  has  a  double  origin,  and  of  course  shows  very  con- 
siderable differences  of  composition  in  different  organs. 


136 


THE  HUMAN  MECHANISM 


B.  MECHANICS  OF  THE  CIRCULATION  OF  THE  BLOOD 

AND    OF    THE    FLOW    OF    LYMPH 

The  greatest  discovery  ever  made  in  physiology  was 
that  of  the  circulation  of  the  blood.  As  late  as  the  settle- 
ment of  the  earliest  English  colonies  in  America  it  was 
thought  that  the  blood  moved  back  and  forth  in  the  blood 
vessels,  as  the  waters  in  the  sea  ebb  and  flow ;  but  of  any 
circulation,  in  the  sense  of  a  steady  stream  returning  to 
its  source,  there  was  no  idea;  and  it  was  not  until  1621 

that  William  Harvey,  an  Eng- 
lish physician,  proved  beyond 
the  shadow  of  a  doubt  that  the 
blood  in  the  body  of  all  the 
higher  animals  flows  like  a 
stream  always  in  one  direc- 
tion, ultimately  returning  to 
its  source. 

Tests   made    upon  various 
FIG.  62.  The   circulation  of  the    animals  have  shown  that  this 
blood  as  seen  in  the  small  ar-    circulation  is  accomplished  in 
teries  and   capillaries   of    the 


web  of  a  frog's  foot 


the  surprisingly  short  time 
of  from  twenty  to  thirty  sec- 
onds ;  which  means  that  the  whole  mass  of  the  blood  (in 
man  about  twelve  pints)  passes  between  three  and  four  thou- 
sand times  a  day  through  the  various  organs  of  the  body, 
bringing  to  them  their  food,  carrying  away  their  wastes, 
and  in  general  helping  to  maintain  normal  conditions.  By 
what  hydraulic  machinery  is  this  marvelous  work  done  ? 

6.    The  Motive   Power  of  the  Circulation  as  a  Whole. 

The  Beat  of  the  Heart. — Whenever  a  mass  of  liquid  is 

kept  in  motion  we  naturally  look  first  for  the  motive  power. 

In  answering  the  question,  "  What  makes  the  blood  cir- 

.culate?  "  we  shall  find  that  while  there  are  several  causes, 


THE  CIRCULATION  OF  THE  BLOOD         137 

one  of  these,  namely  the  beat  of  the  heart,  is  vastly  more 
important  than  all  the  others  combined.  This  fact  is  now 
so  familiar  that  it  is  hard  to  realize  that  we  owe  to  Harvey 
not  only  the  discovery  of  the  circulation,  but  also  the  dis- 
covery of  the  meaning  of  the  heart  beat.  Before  his  time, 
to  be  sure,  the  living  heart  had  been  seen  at  work,  alter- 
nately shrinking  in  size  and  then  swelling,  the  shrinking 
being  called  systole  and  the  swelling  diastole.  But  these 
changes  in  size  were  regarded  as  the  results  of  the  contrac- 
tion and  expansion  of  certain  "  vital  spirits  "  which  the 
arterial  blood  was  then  supposed  to  contain,  and  not  as 
muscular  contractions  and  relaxations.  Harvey  showed 
that  the  heart  is  a  powerful  muscle,  and  that  its  systole  is 
a  muscular  contraction  ;  that  during  systole  it  becomes 
hard,  just  as  the  biceps  muscle  does  when  it  shortens, 
and  during  diastole  soft  and  flabby;  he  also  proved  that 
with  each  systole  the  heart  drives  or  spouts  blood  into  the 
large  arteries  (the  aorta  and  the  pulmonary  artery),  and 
that  this  blood  is  prevented  from  flowing  back  into  the 
heart  during  diastole  by  membranous  valves  at  the  very 
beginning  of  the  large  arteries  in  question. 

7.  The  Heart  a  Muscular  Force-Pump.  —  The  beat  of  the 
heart,  even  to  its  most  minute  detail,  is  one  of  the  most 
important  as  well  as  one  of  the  most  interesting  subjects 
in  physiology ;  everything  in  the  body  hangs  on  its  proper 
efficiency  and  regulation,  and  it  cannot  be  too  thoroughly 
studied.  For  our  present  purposes  it  will  suffice  to  describe 
the  heart  as  composed  essentially  of  a  pair  of  muscular 
force-pumps.  Dissection  shows  that  it  is  divided  into 
right  and  left  halves  (see  Fig.  65),  completely  separated 
from  each  other,  and  that  each  half  consists  of  two  cham- 
bers, —  an  auricle  and  a  ventricle.  The  auricles,  into 
which  the  great  veins  open,  have  thin  muscular  walls  and 
are  comparatively  small  in  size ;  the  ventricles,  on  the 
other  hand,  from  which  the  great  arteries  arise,  have  thick 


138 


THE  HUMAN  MECHANISM 


muscular  walls,  especially  the  left  ventricle.  The  ventricles 
indeed  constitute  the  principal  part  of  the  force-pump; 
the  auricles  merely  facilitate  the  work  of  the  ventricles, 
and  for  purposes  of  elementary  study  may  be  mostly  neg- 
lected. The  student  should,  if  possible,  examine  for  him- 
self and  actually  handle  the  auricles,  ventricles,  and  great 
blood  vessels  of  a  sheep's  heart,  which  in  size  and  struc- 
ture sufficiently  resembles  the  human  heart.  Figures  15 
and  142  should  also  be  consulted. 

8.  The  Mechanics  of  the  Heart  Beat.  —  All  force-pumps 
consist  of  two  indispensable  parts,  —  some  device  for  press- 
ing upon  a  liquid  within  a  chamber,  and  valves  at  the 

I  openings  of  the  cham- 

|  I  ber  so  arranged  as  to 

allow  the  passage  of 

X*^NX  II  ^  ^\  I  I  the  liquid  in  one  di- 
rection only.  Each 
ventricle  of  the  heart 
is  really  such  a  pump, 
and  is  provided  with 
two  sets  of  valves,  — 
one  set  at  the  inlet, 
between  the  auricles 
and  the  ventricles, 
and  the  other  at  the 

arterial  outlet.  These  valves  permit  blood  to  pass  only 
from  the  great  veins  through  the  auricles  and  on  through 
the  ventricles  to  the  great  arteries.  The  contraction  of  the 
muscular  wall  of  the  ventricles  produces  pressure  on  the 
blood  within  their  cavities ;  this  pressure  quickly  and  easily 
closes  the  auriculo-ventricular  valves,  and  finally  forces 
open  the  shut  valves  at  the  openings  of  the  great  arter- 
ies. In  this  way  the  right  ventricle  drives  venous  blood 
into  the  pulmonary  artery,  and  the  left  ventricle  arterial 
blood  into  the  aorta.  With  the  relaxation  of  the  ventricles 


'y//////////, 

'    t  r  ^ 


FIG.  63.  Diagram  of  the  action  of  a 
force-pump 


THE  CIRCULATION  OF  THE  BLOOD 


139 


(diastole)  pressure  falls  within  their  cavities,  and  were  it 
not  for  the  valves  at  the  mouths  of  the  aorta  and  the 
pulmonary  artery,  blood  would  regurgitate,  or  flow  back, 
into  the  heart;  but 
this  "slip"  (as  it  is 
called  in  hydraulics) 
the  valves  prevent, 
and  the  ventricles 
again  fill  through  the 
only  open  channel,  i.e. 
the  one  leading  from 
the  great  veins  and 
the  auricles.  Thus  by 
contractions  rhyth- 
mically repeated  the  FIG.  64.  The  force-pump  action  of  a  ventricle 
heart  continues  to  of  the  heart 

snrmtor  deliver  hlood    On  the  left  is  shown  the  condition  durinS  dias~ 

tole  ;  on  the  right,  during  systole 

from  the  two  sets  of 

great  veins  into  the  two  sets  of  great  arteries.  It  is  plainly 
a  double  force-pump,  or,  better,  a  pair  of  force-pumps  lying 
and  working  side  by  side. 

9.  The  Arterial  and  the  Venous  Reservoirs.  —  To  under- 
stand the  exact  nature  and  result  of  the  work  of  the  heart 
we  must  now  consider  the  relation  of  this  living  pump  to 
the  pipe  system  (arteries,  capillaries,  and  veins)  with  which 
it  is  connected.  The  student  should  first  trace  the  general 
course  of  the  circulation  in  the  simple  diagrammatic  repre- 
sentation given  in  Fig.  65.  This  shows  that  the  blood  which 
enters  the  aorta  from  the  left  ventricle  must  return  to  the 
right  side  of  the  heart  and  pass  through  the  lungs  before  it 
can  again  reach  the  aorta.  As  the  physical  principles  of  the 
circulation  are  the  same  for  the  systemic  and  the  pulmonary 
vessels,  we  shall  confine  our  attention  to  the  former. 

In  the  first  place  we  may  observe  that  the  heart  pumps 
the  blood  into  what  is  practically  a  large  reservoir  (the 


140 


THE  HUMAN  MECHANISM 


FIG.  65. 

Diagram 

of  the  organs 

of  the  circulation 

L,  pulmonary  circulation;  M, circulation  through 
the  organs  suspended  by  the  mesentery,  the 
hlood  being  carried  to  the  liver  P  before  it 
returns  to  the  heart.  The  circulation  through 
other  organs,  such  as  brain,  muscles,  skin, 
and  kidneys,  is  indicated.  Lymphatics  are 
represented  by  dotted  lines 


larger  arteries),  and 
that  the  blood  flows 
from  this  reservoir  to 
a  second  reservoir  (the 
larger  veins)  l>y  vari- 
ous routes;  for  the 
vessels  of  the  different 
organs  represent  many 
alternative  courses 
which  the  blood  may 
take  in  flowing  from 
the  arterial  to  the  ve- 
nous reservoir.  The 
blood  stream  indeed 
may  be  compared 
with  a  stream  supply- 
ing water  power  to  a 
series  of  mills  in  a 
manufacturing  town. 
The  larger  arteries 
from  the  main  source 
of  pressure  (the  heart) 
correspond  to  the 
headrace  from  above 
the  dam,  while  the 
larger  veins  corre- 
spond to  the  tailrace. 
The  water  flows  from 
the  one  into  the  other 
only  through  the 
smaller  sluices,  or 
penstocks,  which  sup- 
ply the  mills.  So,  in 
the  vascular  system, 
a  part  of  the  blood 


To  head  and 
neck 

To  shoulder  and 
arm 


To  stomach, 
intestine,  etc. 

To  kidney 


To  leg 


FIG.  66.  The  aorta  and  its  main  branches 

At  the  beginning  are  shown  the  three  pocket  valves,  which  prevent  regurgi 
tation  of  blood  during  diastole 


141 


142  THE  HUMAN  MECHANISM 

pumped  into  the  arterial  reservoir,  or  aorta,  finds  its  way 
into  the  venous  reservoir  by  way  of  the  skin,  another  part 
by  way  of  the  digestive  organs,  another  by  way  of  the 
brain,  still  another  by  way  of  the  kidneys,  and  so  on ;  but 
the  flow  in  every  case  is  essentially  the  same,  namely  from 
a  reservoir  of  high  pressure  to  one  of  lower  pressure. 

10.  The  Driving  Force  for  the  Flow  of  Blood  from  the 
Aorta.  Pressure  in  Arteries  and  Veins.  —  The  hydraulic 
conditions  in  the  aorta  may  be  illustrated  by  means  of 
the  following  simple  piece  of  apparatus.  To  an  ordinary 
rubber  syringe  attach  a  piece  of  elastic  rubber  tubing,  the 
other  end  of  which  is  closed  by  a  detachable  nozzle.  If 
now  the  nozzle  be  removed  and  water  pumped  into  the 
tube,  it  will  be  found  that  the  flow  from  the  open  end  con- 
sists of  squirts  or  spouts  and  continues  only  during  the 
stroke  of  the  pump ;  if,  however,  we  attach  the  nozzle, 
and  again  pump  water  into  the  tube,  the  resistance  caused 
by  the  small  orifice  of  the  nozzle  prevents  the  water  from 
flowing  out  of  the  tube  as  fast  as  the  syringe  pumps  it  in. 
The  tubing  becomes  distended  with  water.  Since,  however, 
the  tube  is  elastic,1  and  so  tends  to  return  to  its  original 
size,  it  forces  the  liquid  out  through  the  nozzle  even  be- 
tween the  strokes  of  the  pump.  The  immediate  cause  of 
the  steady  flow  from  the  nozzle  is  therefore  the  elasticity 
of  the  rubber  tube.  The  intermittent  stroke  of  the  pump 
produces  distention  of  the  tube,  and  the  elasticity  of  the  dis- 
tended tube  constantly  forces  the  water  out  of  the  nozzle. 

Closely  similar  conditions  obtain  in  the  arterial  reser- 
voir. Here  the  outlet  is  also  through  very  small  tubes, 

1  An  elastic  body  is  one  which  returns  to  its  original  shape  when  it  has 
been  stretched,  compressed,  or  otherwise  deformed.  Elasticity  must  not 
be  confounded  with  "extensibility,"  or  the  property  of  allowing  stretch- 
ing. Thus  when  we  "pull"  taffy  we  deal  with  a  body  which  is  very 
extensible  but  which  is  practically  inelastic.  A  body  indeed  may  be  exten- 
sible only  with  difficulty,  but  possess  a  very  high  degree  of  elasticity; 
ivory  is  a  good  example  of  this  kind. 


THE  CIRCULATION  OF  THE  BLOOD         143 

the  small  arteries,  whose  bore  is  not  greater  than  ^  or 
yl-Q  of  an  inch;  which  fact  introduces  the  same  condition 
as  does  the  nozzle  of  our  apparatus,  i.e.  a  resistance  to  the 
outward  flow  of  the  blood.  Consequently  the  blood  can- 
not flow  out  of  the  aorta  as  rapidly  as  it  is  driven  in,  and 
the  extensible  and  elastic  walls  are  necessarily  stretched. 
The  immediate  effect  of  the  heart  beat  is  to  keep  the 
arterial  reservoir  overfilled  or  distended,  so  that  the  elastic 
reaction  of  its  walls  is  brought  into  play ;  and  it  is  this 
elastic  reaction  of  the  arterial  walls  which  is  the  imme- 
diate cause  of  the  steady  outflow  through  the  small  arter- 
ies and  capillaries. 

The  force  of  compression,  or  pressure,  exerted  by  the 
elastic  arterial  walls  is  primarily  exerted  upon  the  blood 
within  them ;  and  the  more  the  arteries  are  distended  the 
greater  will  be  the  pressure  exerted  on  the  blood.  A  liquid 
thus  under  pressure  tends  to  find  an  outlet;  should  any 
part  of  the  arterial  wall  be  weak,  as  sometimes  happens 
in  diseased  conditions,  it  is  bulged  outward ;  and,  for  the 
same  reason,  a  flow  of  blood  will  take  place  through  such 
outlets  as  are  presented  by  the  smaller  arteries  and  capil- 
laries. Moreover,  the  greater  the  pressure  of  the  blood  in 
the  arteries,  the  more  rapid  will  be  the  flow  into  the  capil- 
laries. Hence  it  is  customary  to  use  the  arterial  blood 
pressure  as  a  measure  of  the  force  of  elasticity  exerted 
by  the  distended  arterial  wall. 

The  veins,  on  the  other  hand,  are  less  elastic  than  the 
arteries;  they  are  indeed  more  like  mere  conducting 
tubes  through  which  the  blood  can  flow  back  to  the  heart. 
They  are  not  overfilled  (since,  for  one  reason,  there  is  no 
resistance  to  the  flow  of  blood  out  of  them  into  the  heart), 
and  hence  venous  blood  pressure  is  low. 

Thus  we  have  the  conditions  favorable  for  the  flow  from 
the  aorta  to  the  great  veins,  —  a  high  pressure  in  the  arte- 
rial reservoir  and  a  low  pressure  in  the  venous  reservoir. 


144 


THE  HUMAN  MECHANISM 


It  is  the  function  of  the  heart,  by  continually  pumping  the 
blood  from  the  veins  into  the  arteries,  to  keep  the  arterial 
reservoir  distended,  thus  maintaining  a  difference  of  pres- 
sure in  the  two  reservoirs.  It  is  this  difference  of  pressure 
which  drives  the  blood  through  the  organs. 

11.  The  Distribution  of  the  Blood  among  the  Organs.  - 
Some  organs  require  more  blood  than  others,  and  the  same 

organ  often  requires  more 
blood  at  one  time  than  at 
another.  Thus  muscles  and 
glands,  the  seat  of  very 
active  chemical  changes, 
require  more  blood  than 
C  a  tendon ;  and  a  gland  re- 
quires more  blood  during 
the  process  of  secretion 
than  during  rest.  How  is 
the  supply  of  blood  to  the 
organs  regulated  to  meet 
B  their  varying  needs?  In 
the  first  place,  some  or- 
gans are  more  vascular 
than  others ;  those  requir- 
ing a  larger  supply  of 
blood  receive  a  greater 
number  of  arteries  from 
the  arterial  reservoir  and 
have  a  closer  network  of 
capillaries.  But  in  addi- 
tion to  this,  these  smaller 


FIG.  67.  Cross  sections  of  portions  of 
the  wall  of  a  smaller  artery  (a)  and 
a  smaller  vein  («) 

-4,  internal  coat;  B,  middle  coat,  with 
muscle  fibers ;  C,  outer  coat  of  connect- 
ive tissue.  The  contraction  of  the  cir- 
cularly disposed  muscle  fibers  narrows 
the  bore  of  the  tube 


arteries    contain   circular 
muscle  fibers  whose  con- 
traction diminishes  the  bore  of  the  tube.    When  an  organ 
needs  more  blood,  the  muscle  fibers  of  its  small  arteries 
relax,  thus  permitting  the  arterial  tubes  to  widen  or  dilate ; 


THE  CIRCULATION  OF  THE  BLOOD         145 

just  as  when  we  want  the  water  to  flow  faster  from  a 
faucet,  by  turning  the  spigot  a  little  further  we  widen  the 
outlet  from  the  pipe.  When  less  blood  is  needed  the 
small  arteries  are  caused  to  constrict,  just  as  a  spigot  may 
be  partially  turned  off  (see  Sections  25-27).  In  this  way 
the  flow  of  blood  to  any  organ  is  regulated  to  meet  the 
varying  needs  of  the  organ  in  question.1 

12.  Secondary  Aids  to  the  Circulation.  —  In  the  preced- 
ing discussion  we  have  seen  that  the  cause  of  the  flow  of 
blood  through  the  organs  is  the  difference  of  pressure  in 
the  two  reservoirs.  We  have  further  seen  that  this  differ- 
ence of  pressure  is  maintained  by  the  heart  beat  in  pump- 
ing blood  from  the  venous  into  the  arterial  reservoir.  A 
moment's  consideration  will  show  that  anything  which 
hastens  the  flow  of  blood  from  the  veins  into  the  heart, 
and  so  lowers  pressure  within  the  veins,  would  similarly 
aid  the  circulation,  since,  with  the  same  arterial  pres- 
sure, more  blood  will  flow  into  an  empty  vein  than  into 
one  which  is  partially  filled. 

(a)  The  Breathing  Movements.  —  There  are  two  factors 
which  thus  tend  to  empty  the  veins.  The  first  is  the 
suction  exerted  on  the  blood  within  the  veins  by  breathing 
movements.  The  exact  mechanism  by  which  this  is  accom- 
plished must  be  left  for  consideration  in  the  chapter  on 
respiration.  Suffice  it  to  say  here  that  just  as  the  enlarge- 
ment of  the  thorax,  when  we  take  in  a  breath,  sucks  air  into 
the  lungs,  so  it  also  sucks  blood  from  the  large  veins  outside 

1  In  order  that  the  student  may  become  more  familiar  with  these  fun- 
damental hydraulic  principles  of  the  circulation,  such  questions  as  the 
following  should  be  answered :  (1)  What  are  the  two  principal  factors 
whose  variations  change  the  amount  of  arterial  pressure?  Illustrate 
by  an  example  or  model.  (2)  How  would  the  dilation  of  all  the  arteries 
of  the  intestine  affect  the  general  arterial  pressure  ?  (3)  What  would  be 
the  effect  upon  the  amount  of  blood  flowing  through  the  skin  under  this 
condition  ?  (4)  How  would  dilation  of  the  arteries  of  the  skin  affect  the 
blood  flow  through  the  brain  ? 


146 


THE  HUMAN  MECHANISM 


the   thorax  into  those    which  lie  within   it;    because  of 
the  thickness  of  the  walls  of  the  arteries,  the  same  effect 

occurs  to  only  a  very 
slight  extent  in  the 
arterial  reservoir. 
During  expiration, 
on  the  other  hand, 
the  reduction  in  size 
of  the  thorax  forces 
air  out  of  the  lungs, 

FIG.  68.  The  pocket  valves  in  the  veins 


On  the  right  is  shown  the  external  appearance 
of  the  vein  at  the  valves  when  the  latter  are 
closed ;  on  the  left  a  vein  slit  lengthwise  and 
opened ;  in  the  middle  a  longitudinal  section 
of  a  vein 


and  we  might  expect 
that  it  would  simi- 
larly force  blood 
from  the  veins  within 
the  thorax  into  those 
without.  And  this  it  certainly  would  do  if  the  veins  were 
not  provided  with  valves  which  allow  the  blood  to  flow 
only  toward  the  heart.  In  general,  therefore,  both  inspira- 
tion and  expiration  aid  the  circulation,  the  former  by 
sucking  blood  into  the  thoracic  veins  and  so  emptying 
those  outside,  the  latter  by  making  this  blood  in  the  intra- 
thoracic  veins  flow  on  more  rapidly  to  the  heart,  whence  it 
is  pumped  into  the  arteries.  In  a  word,  deep  breathing 
greatly  promotes  a  good  circulation. 

(b)  Intermittent  Compression  of  the  Veins.  —  The  other 
secondary  factor  of  the  circulation  is  intermittent  compres- 
sion of  the  veins,  and  in  ordinary  life  this  is  brought  about 
in  two  ways :  (1)  Whenever  a  muscle  contracts  it  thickens 
and  hardens ;  the  veins  and  capillaries  which  are  between 
the  fibers  and  fiber  bundles,  or  in  the  connective  tissue 
between  two  contracting  muscles,  will  thus  have  the  blood 
squeezed  out  of  them  into  the  large  veins ;  when  the  mus- 
cle relaxes  the  empty  veins  and  capillaries  will  readily 
fill  from  the  arteries,  since  the  valves  of  the  veins  will 
prevent  any  backward  flow  of  the  blood  from  the  larger 


THE  CIRCULATION  OF  THE  BLOOD         147 

veins.  Alternate  contractions  and  relaxations  of  muscles, 
therefore,  aid  the  flow  of  blood  through  this  so-called 
"  pumping  "  action  on  the  veins.  (2)  A  similar  pumping 
action  on  the  veins  is  exerted  by  alternate  flexions  (bend- 
ings)  and  extensions  at  any  joint.  In  general,  flexions 
force  the  blood  out  of  the  veins,  while  extensions  allow 
them  to  fill.  When  we  remember  how  largely  most  of  our 
usual  muscular  actions  consist  of  alternate  flexions  and 
extensions  of  joints,  and  alternate  contractions  and  relaxa- 
tions of  muscles,  as,  for  example,  in  walking  and  running, 
we  can  at  once  appreciate  how  greatly  muscular  activities 
must  aid  the  circulation.  When  to  the  effect  of  these  we 
add  the  suction  action  of  the  deepened  breathing  move- 
ments the  effect  upon  the  circulation  becomes  very  great. 

13.  Massage.  —  The  action  of  massage  is  only  another 
illustration  of  the  same  principle.    By  rubbing  the  legs  and 
arms  in  the  direction  of  the  heart,  the  blood  contained 
in  their  veins  is  forced  onward  and  the  circulation  aided, 
precisely  as  when  a  muscle  contracts  or  one  member  of 
a  limb  is  flexed  upon  another. 

14.  The  Lymphatics.  —  Important  as   are  the  suction 
action  of  the  breathing  movements  and  the  pumping  action 
of  contracting  muscles  as  aids  to  the  circulation  of  the 
blood,   they  are   even  more  important  as   causes  of  the 
flow  of  lymph  along  the  greater  lymphatic  trunks  toward 
the  heart.    Reference  to  the  general  method  of  origin  of 
lymphatics,  as  described  in  Chapter  III,  will  show  that 
the  lymph  in  the  lymph  spaces,  unlike  the  blood  in  the 
capillaries,  has  not  behind   it   a  high-pressure  reservoir; 
there  is   no  such  force  from  behind  to   send  it  onwards, 
since  the  lymphatics  arise  blindly  in  the  tissues.    What, 
then,  makes  the  lymph  flow  along  the  lymphatics  toward 
the  heart? 

The  lymphatics  resemble  the  veins  in  structure,  having 
thin  walls  and  pocket  valves ;  like  the  veins,  most  of  them 


148  THE  HUMAN  MECHANISM 

originate  in  extra-thoracic  organs,  and  join  or  combine  to 
form  larger  trunks  as  they  proceed  toward  the  thorax. 
All  of  them  finally  unite  in  two  large  lymphatics  within 
the  thoracic  cavity,  and  these  open  into  the  great  veins 
near  the  heart.  (Figures  30  and  65  should  be  consulted 
in  this  connection.)  It  is  at  once  clear  that  the  breathing 
movements  must  exert  on  the  lymph  within  these  thin- 
walled  vessels  exactly  the  same  suction  action  as  they 
exert  on  the  blood  in  the  veins;  and  anything  which 
increases  this  suction  action,  such  as  the  deepened  breath- 
ing movements  during  muscular  activity,  must  necessa- 
rily increase  the  flow  of  lymph  from  every  organ  of  the 
body.  On  the  other  hand,  a  pumping  action  on  the  lymph 
in  the  organs  results  from  all  rhythmic  movements  of  parts 
of  the  body  with  reference  to  one  another,  since  each 
change  of  position  carries  with  it  some  change  of  external 
pressure  on  lymphatics.  Familiar  examples  are  the  move- 
ments of  arms  and  legs  in  locomotion,  of  the  diaphragm  in 
breathing,  and  of  the  lungs  in  respiration. 

It  has  also  been  supposed  that  a  third  cause  of  the 
lymph  flow  is  the  passage  of  waves  of  constriction  (peri- 
stalsis, cf.  p.  122)  over  the  larger  lymphatics.  This,  how- 
ever, probably  plays  only  a  minor  part. 

Finally,  in  the  formation  of  lymph  from  the  blood,  more 
water  generally  passes  from  the  capillaries  to  the  lymph 
spaces  than  from  the  lymph  spaces  into  the  capillaries. 
Under  these  circumstances,  at  least  at  certain  times,  the 
lymph  spaces  become  distended  and  a  certain  low  pressure 
obtains  in  them.  This  we  may  speak  of  as  the  "active 
force "  of  lymph  formation,  and  it  constitutes  a  fourth 
factor  in  causing  the  lymph  flow. 

We  have  already  pointed  out  the  importance  of  the 
lymph  flow  in  maintaining  the  lymph  currents  about  the 
living  cells ;  we  are  now  able  to  appreciate  the  importance 
of  those  agents  which  secure  this  flow.  As  enumerated 


THE  CIRCULATION  OF  THE  BLOOD         149 

above,  they  are  four  in  number :  (1)  suction  action  of  the 
breathing  movements  ;  (2)  pumping  action  of  muscular  or 
passive  movements  ;  (3)  active  force  of  lymph  formation  ; 
(4)  peristaltic  contractions  of  the  large  lymphatics. 

Of  these  the  fourth  is  at  least  doubtful  and  in  no  case 
of  great  importance ;  the  other  three  may  therefore  be 
regarded  as  the  chief  causes  of  the  lymph  flow,  and  of 
these  the  first  and  second  are  brought  into  most  effective 
action  by  muscular  activity;  this  deepens  the  breathing 
movements  and  so  increases  their  suction  action  on  the 
lymph,  while  the  movements  of  the  body  exert  on  the 
lymphatics  a  pumping  action,  which  is  largely  lacking 
during  complete  inactivity.  The  great  practical  impor- 
tance of  this  aspect  of  the  subject  will  be  discussed  beyond 
in  those  chapters  which  deal  with  the  hygiene  of  muscular 
activity  (Part  II). 

O.   THE   ADJUSTMENT  OF   THE   CIRCULATION   TO   THE 
NEEDS  OF  EVERYDAY  LIFE 

The  total  quantity  of  blood  in  the  body  (ten  to  fourteen 
pints)  is  not  enough  to  furnish  a  working  supply  to  all 
organs  at  the  same  time ;  and  since,  in  general,  whenever 
an  organ  works  it  receives  more  blood,  and  when  it  is  at 
rest  it  receives  less,  our  daily  life  with  its  changes  of  activ- 
ity among  the  organs  makes  necessary  frequent  adjustments 
of  the  circulation  to  the  needs  of  the  organs  at  various 
times. 

Some  of  these  adjustments  are  matters  of  familiar  experi- 
ence. The  increased  flow  of  blood  to  the  skin  on  a  warm 
day  makes  the  veins  stand  out  and  the  face  red,  and  we 
are  conscious  of  the  more  rapid  heart  beat  during  muscu- 
lar activity,  even  in  an  act  so  simple  as  running  upstairs. 
Other  adjustments  are  not  so  evident,  but  betray  themselves 
by  their  results,  as  happens  after  a  hearty  meal  when  the 


150  THE  HUMAN  MECHANISM 

demand  of  the  digestive  organs  for  blood  lessens  the  sup- 
ply to  the  brain  and  we  feel  disinclined  to  hard  mental 
work.  We  may  begin  our  study  of  these  adjustments  by 
learning  what  occurs  in  the  circulation  during  some  of  the 
more  common  activities  and  events  of  daily  life. 

15.  The  Circulation  during  Exposure  to  Heat  and  Cold. 
—  When  the  skin  is  exposed  to  cold  its  blood  supply  is 
greatly  diminished ;  the  veins  no  longer  stand  out  promi- 
nently on  the  hand,  and  if  a  small  area  of  skin  be  made 
pale  by  pressing  upon  it  (thus  driving  the  blood  out  of 
its  capillaries),  the   pallor  passes  off  very  slowly.    This 
simple  experiment  shows  that  blood  is  flowing  but  slowly 
from  the  arterial  reservoir  into  the  skin.    Conversely,  on 
a  warm  day  the   veins  stand   out  prominently  and  the 
red  color  instantly  returns  upon  the  removal  of  pressure. 
These  variations  in  the  supply  to  the  skin  are  due,  as  we 
have  already  seen  (p.  144),  to  changes  in  the  diameter  of 
the  arteries  of  the  skin,  which  changes  serve  like  the  spigot 
of  an  ordinary  water  faucet  to  regulate  the  flow  of  liquid. 

The  changes  in  the  blood-flow  through  the  skin  are 
accompanied  by  corresponding  but  inverse  changes  in  the 
internal  organs.  On  a  cold  day  the  stomach  and  intes- 
tines, the  pancreas,  the  liver,  the  kidneys,  etc.,  are  richly 
supplied  with  blood,  while  on  a  warm  day  their  blood 
supply  is  diminished.  In  the  former  case  the  blood  with- 
held from  the  skin  finds  its  way  into  the  internal  organs ; 
in  the  latter  case  the  skin  draws  upon  these  organs  for 
its  needed  supply.  The  circulation  in  the  internal  organs 
compensates  for  that  in  the  skin. 

16.  The  Reason  for  Compensatory  Changes We  have 

seen  that  it  is  the  function  of  the  heart  to  keep  the  arte- 
rial reservoir  adequately  distended  with  blood,  thus  sup- 
plying a  steady  driving  force  for  the  flow  of  blood  through 
the  organs.    When  the  small  arteries  of  the  skin  widen 
on  a  warm  day  blood  escapes  more  rapidly  into  the  skin 


THE  CIRCULATION  OF  THE  BLOOD          151 

from  the  arterial  reservoir.  This  alone  would  diminish 
the  amount  of  blood  in  the  reservoir  unless  the  heart 
pumped  more  blood,  or  unless  the  dilation  or  widening  of 
the  cutaneous  arterioles  were  compensated  by  a  constriction 
elsewhere,  so  that  the  total  drain  on  the  reservoir  remained 
the  same.  In  the  case  in  question  it  is  the  latter  of  these 
alternatives  which  is  adopted,  and  the  reservoir  is  kept 
filled  without  calling  on  the  heart  to  pump  more  blood. 

Conversely,  on  a  cold  day  the  diminution  of  the  outflow 
into  the  skin  would  lead  to  a  backing  up  or  accumulation 
of  blood  in  the  great  arteries,  and  so  to  their  increased  and 
perhaps  undesirable  distention,  if  the  dilation  of  the  arte- 
rioles of  internal  organs  did  not  provide  an  outlet  for  the 
surplus  blood. 

Nowhere,  perhaps,  is  this  principle  of  compensatory  dila- 
tion or  constriction  of  arteries  in  one  region,  to  allow  for 
the  effect  of  the  opposite  change  in  some  other  region, 
so  highly  developed  or  so  fully  applied  as  in  the  reactions 
of  the  body  to  changes  in  external  temperature. 

17.  The  Circulation  during  Muscular  Activity.  —  Dur- 
ing muscular  activity  the  arterioles  of  the  muscles  and  of 
the  skin  are  dilated,  the  former  in  order  to  supply  more 
blood  to  the  working  organ,  the  latter  to  aid  in  the  dis- 
charge of  the  excess  of  heat  produced  by  the  contracting 
muscles.  The  heavy  drain  upon  the  arterial  reservoir  by 
these  two  large  areas  (among  the  largest  in  the  body)  is 
compensated  to  some  extent  by  the  constriction  of  the 
arteries  of  the  digestive  and  other  internal  organs.  This 
alone,  however,  would  not  suffice  to  keep  the  arterial  reser- 
voir filled;  and  we  accordingly  find  that  the  heart  beats 
more  rapidly  and  more  powerfully,  pumping  more  blood 
into  the  aorta  in  a  given  time. 

It  is  very  important  to  remember  that  muscular  activ- 
ity is  the  one  condition  of  life  which  materially  increases 
the  work  of  the  heart ;  at  other  times  the  greater  demand 


152 


THE  HUMAN  MECHANISM 


of  blood  for  the  working  organ  is  met  more  or  less  success- 
fully by  withdrawing  blood  from  a  resting  organ,  while  the 
supply  to  the  whole  arterial  system,  and  hence  the  work 
of  the  heart,  remains  approximately  unchanged.  During 
muscular  exercise,  and  then  only,  is  the  heart  called  upon 
for  decidedly  increased  work  ;  and,  like  the  skeletal  muscle, 
its  strength,  its  ability  to  meet  strain  and  emergencies,  and 
to  withstand  fatigue  depend  to  a  great  extent  upon  the 
training  given  it  in  this  way. 

Muscular  activity  also  influences  the  circulation  indi- 
rectly by  increasing  the  action  of  its  secondary  driving 
forces,  —  the  suction  action  of  the  respiratory  movements 
and  the  pumping  action  of  the  contracting  muscles  on  the 


FIG.  69.  Simple  apparatus  to  illustrate  the  relation  between  the  output  of 
the  heart,  the  peripheral  resistance,  and  the  general  arterial  pressure 

The  amount  delivered  by  the  faucet  represents  the  output  of  the  heart,  and  is 
one  factor  in  keeping  up  arterial  pressure ;  two  alternative  routes  of  out- 
flow, each  capable  of  regulation,  represent  the  arterioles  to  different 
organs.  Compensatory  constrictions  and  dilations  and  other  hydraulic 
conditions  described  in  the  text  may  readily  be  imitated 


THE  CIRCULATION  OF  THE  BLOOD         153 

veins.  These  are  among  the  most  important  effects  of  this 
agent  upon  the  flow  of  blood,  but  they  are  too  complicated 
for  detailed  discussion  here. 

It  is  sometimes  stated  that  muscular  exercise  "quickens" 
the  circulation.  This  is  true  in  the  sense  that  the  heart 
pumps  more  blood  into  the  pulmonary  artery  and  the  aorta 
than  during  rest.  From  this  it  follows  that  during  exercise 
more  blood  flows  through  the  lungs,  and  that  blood  flows 
more  rapidly  out  of  the  arterial  reservoir  ;  but  it  does  not 
mean  that  blood  flows  more  rapidly  through  all  organs, 
for^the  digestive  and  other  internal  organs  at  such  times 
actually  receive  less  blood.  Indeed,  we  may  say  that  the 
quickening  of  the  circulation  during  exercise  is  chiefly  con- 
fined to  three  important  organs,  —  the  muscles,  the  skin, 
and  the  lungs ;  in  other  organs  the  change  is  relatively 
slight,  as,  for  example,  in  the  brain ;  while  in  still  others, 
notably  those  of  the  digestive  system  and  the  kidneys,  the 
speed  is  diminished. 

18.  The  Circulation  during  Sleep.  —  An  adequate  blood 
supply  is  necessary  to  the  full  activity  of  the  brain ;  when 
the  circulation  in  this  organ  is  seriously  interfered  with, 
imperfect  mental  action  or  even  unconsciousness  is  a 
result.  Thus  when  all  the  arterioles  of  the  body  dilate, 
or  the  heart  beat  is  slowed  down,  in  consequence  of  some 
sudden  "  shock,"  so  that  pressure  in  the  arterial  reservoir 
falls  too  far,  the  driving  force  for  the  flow  of  blood  through 
the  brain,  in  common  with  other  organs,  is  diminished, 
and  the  person  loses  consciousness,  or  faints.  Most  cases 
of  fainting  are  traceable  to  one  or  the  other  of  these  causes. 

The  most  familiar  and  most  common  example  of  uncon- 
sciousness, however,  is  that  of  sleep,  which  in  so  many 
respects  resembles  fainting  as  to  suggest  that  the  uncon- 
sciousness in  both  cases  is  due  to  the  same  cause,  namely, 
a  lessened  blood  supply  to  the  brain.  Unquestionably, 
the  amount  of  blood  flowing  through  the  brain  is  greatly 


154 


THE  HUMAN  MECHANISM 


lessened  during  sleep.  The  evidence  for  this  statement 
cannot  be  given  here  in  full,  but  it  is  known  that  where 
accident  has  destroyed  a  part  of  the  rigid  bone  of  the 

skull,  and  the  wound  has 
been  covered  over  by  con- 
nective tissue  and  skin, 
the  scar  sinks  in  during 
sleep,  --  indicating  less 
blood  in  the  brain,  —  and 
returns  to  the  level  of 
the  general  surface  of  the 
head  when  the  subject 
awakens. 

Upon  this  point  of  di- 
minished blood  supply  to 
the  brain  during  sleep  al- 
most all  physiologists  are 
agreed ;  there  is  also  gen- 
eral agreement  that  the 
arm  and  the  leg  increase 
in  volume  when  we  go  to 

sleep,  and  this  is  thought 
FIG.  70.  Showing  the  relation  between  6 

general  arterial  tone  and  the  supply    to  be  due  to  a  dilation  of 
of  blood  to  the  brain  the  arteries  of  the  skin. 

In  A  the  arterioles  of  the  organs  m,  n,  s  It  is   very  significant,  On 

are  constricted,  raising  general  arterial  ,  i          ,1         i_        i     xu 

pressure  which  forces  a  large  amount  the   °ther  hand'    that  the 

of  blood  through  the  brain  6.    In  B  the  arm    shrinks    in    volume 

genTra^arterial   pressure  fcTlow,  and    wnen   tne    brain   is  active 

less  blood  is  forced  through  the  brain,  in  mental  work,  and  espe- 
cially in  mental  work  in- 
volving the  personal  interest  or  mental  concentration  of 
the  subject  of  the  experiment. 

It  is  thought  by  some  that  other  vascular  areas,  that 
of  the  abdominal  cavity,  for  example,  behave  in  this 
respect  in  the  same  way  as  the  skin;  but  on  this  point 


THE  CIRCULATION  OF  THE  BLOOD         155 

the  evidence  is  not  conclusive.  It  is,  indeed,  not  improb- 
able that  these  other  vascular  areas  play  some  part  in  the 
regulation  of  the  flow  to  the  brain,  but  it  is  not  likely  that 
they  stand  in  the  same  intimate  relation  to  it  as  does  the 
skin. 

The  fact  is  clear,  however,  that  a  close  relation  exists 
between  cutaneous  circulation  and  the  maintenance  of 
proper  vascular  conditions  in  the  brain.  Mental  work, 
for  example,  is  more  difficult  for  most  people  in  very 
warm  weather  because  at  that  time  the  cutaneous  arte- 
rioles  are  widely  dilated;  and,  on  the  other  hand,  it  is 
easy  to  understand  why  the  constriction  of  the  vessels  of 
the  skin  by  cold  makes  it  difficult  to  go  to  sleep  with- 
out sufficient  bedclothing. 

19.  The  Circulation  during  the  Digestion  of  a  Meal.  - 
After  eating  a  meal,  more  blood  is  needed  in  the  secret- 
ing digestive  glands  (especially  the  stomach  and  pancreas), 
and  also  in  the  intestinal  organs  of  absorption,  the  villi. 
And  this  need  is  greatest  during  the  first  hour  or  two, 
when  there  is  the  largest  amount  of  food  to  be  worked 
upon.    We   find,  accordingly,  that  the   arteries  of  these 
organs  then  dilate  to  such   an  extent  that  the  mucous 
membrane   of  the  stomach   and  intestine,   which  is  pale 
pink  while  those  organs  are  at  rest,  now  becomes  very 
red  on  account  of   the   large  amount   of   blood  flowing 
through  them. 

There  is  probably  some  compensation  for  this  in  other 
organs,  but  it  is  an  imperfect  compensation.  The 
drowsiness  which  is  apt  to  come  on  after  a  hearty  meal 
is  probably  an  indication  that  these  compensations  are  not 
complete,  and  that  owing  to  the  fall  of  arterial  pressure 
the  brain  is  not  receiving  its  normal  blood  supply. 

20.  Some  Practical  Applications.  —  We  may  pause  here 
to  consider  some  important  practical  applications  of  these 
facts.    While    the   most  active   secretion    is    in   progress 


156  THE  HUMAN  MECHANISM 

nothing  should  be  done  which,  will  take  blood  away  in  large 
quantities  from  the  stomach.  Muscular  exercise,  for  exam- 
ple, then,  as  always,  dilates  the  arterioles  of  the  muscles 
and  skin  and  constricts  those  of  the  digestive  organs;  this 
is  obviously  an  unfavorable  vascular  condition  for  the  act 
of  secretion ;  if  the  meal  be  a  light  one,  so  that  compara- 
tively little  of  the  digestive  juices  are  required,  no  harm 
may  be  done  by  taking  exercise  after  a  meal ;  but  where 
the  meal  is  heavier  it  is  almost  always  unsafe,  especially 
in  warm  weather.  Similar  considerations,  which  are  like- 
wise in  full  accord  with  experience,  indicate  that  it  is 
unwise  to  eat  as  large  meals  in  very  warm  weather  as  in 
cooler  weather ;  the  larger  the  meal  the  greater  the  amount 
of  gastric  juice  required  to  start  its  digestion;  but  in 
warm  weather  the  arteries  of  the  stomach  and  intestine 
tend  to  be  constricted  (see  p.  150),  so  that  it  is  difficult 
to  secure  an  adequate  blood-flow  through  these  organs, 
and  their  efficiency  is  to  this  extent  impaired. 

It  is  sometimes  stated  that  mental  work  immediately 
after  meals  causes  indigestion  by  taking  blood  away  from 
the  digestive  organs  and  sending  it  to  the  brain.  It  is 
very  doubtful,  however,  whether  the  increased  blood-flow 
to  the  brain  is  secured  largely  at  the  expense  of  that  to 
the  digestive  organs.  While  instances  might  be  cited  of 
indigestion  among  people  who  do  mental  work  upon  a 
"full  stomach,"  it  must  be  remembered  that  these  are 
usually  people  who  fail  to  take  proper  exercise  or  suffi- 
cient sleep  and  rest ;  the  indigestion  from  which  they  too 
frequently  suffer  is  more  often  attributable  to  these  causes 
than  to  the  fact  that  the  digestive  organs  are  deprived  of 
their  proper  blood  supply.  . 

21.  The  Mechanism  of  the  Regulation  of  the  Flow  of 
Blood.  —  Having  thus  considered  exactly  what  takes  place 
in  the  circulation  during  some  of  the  more  important 
events  of  daily  life,  we  may  next  inquire  briefly  into  the 


THE  CIRCULATION  OF  THE  BLOOD         157 

physiological  mechanism  by  which  these  adjustments  are 
secured.  Its  most  important  features  are  the  regulation  of 
the  inflow  from  the  heart  into  the  arterial  reservoir  and 
the  regulation  of  the  outflow  through  the  arterioles  and 
capillaries  of  the  organs.  These  two  must  be  adjusted  to 
each  other  in  order  that  the  reservoir  may  remain  full, 
and  thus  the  driving  force  for  the  flow  through  the  organs 
be  maintained.  We  shall  go  into  the  details  of  this  very 
beautiful  but  complicated  mechanism  only  far  enough  to 
enable  the  student  to  appreciate  certain  principles  of  fun- 
damental importance  in  the  practical  conduct  of  life. 

22.  The  Regulation  of  the  Pumping  Action  of  the  Heart. 
-  The  amount  of  blood  which  the  heart  pumps  varies 

considerably  from  time  to  time.  At  times  it  may  be  as 
low  as  three  quarts  a  minute,  and  at  other  times  as  high 
as  twelve  quarts,  the  quantity  being  largely  determined 
by  the  drain  made  at  the  time  upon  the  arterial  reservoir. 
It  will  be  seen  at  once  that  this  involves  a  wide  range  of 
adjustment. 

The  beat  of  the  heart  is  primarily  due  to  events  which 
take  place  within  the  heart  itself.  We  have  seen  that  this 
beat  is  a  muscular  contraction.  But  the  cardiac  muscle 
differs  from  the  skeletal  muscle  in  that  it  does  not  require 
an  impulse  from  the  central  nervous  system  to  throw  it 
into  activity.  When  the  heart  is  cut  off  from  connec- 
tion with  the  rest  of  the  body  it  continues  to  beat  for  a 
time,  and  if  supplied  with  warm  blood  it  may  be  kept 
beating  for  hours.  We  express  this  by  saying  that  the 
heart  beat  is  automatic,  by  which  we  mean  that  the  heart 
contains  within  itself  a  complete  mechanism  for  doing  its 
own  work. 

23.  The  Augmentor  and  the   Inhibitory  Nerves  of  the 
Heart.  —  Nevertheless,  the  heart  receives  from  the  central 
nervous  system  two  pairs  of  nerves,  which  are  able  to  influ- 
ence the  rate  and  the  force  of  the  automatic  beats.    One 


158  THE  HUMAN  MECHANISM 

pair  of  these  nerves  carries  from  the  spinal  cord  to  the 
heart  impulses  which  stimulate  that  organ  to  beat  more 
rapidly  or  more  forcibly,  or  both.  Hence  these  are  known 
as  the  augmentor  or  accelerator  nerves. 

The  fibers  of  the  other  pair  of  nerves  produce  exactly 
the  opposite  effect.  Running  from  the  lower  part  of  the 
brain,  they  carry  to  the  heart  impulses  which  slow  the 
beat  or  lessen  its  force,  or  they  may  produce  both  effects 
at  the  same  time.  They  act,  as  it  were,  like  a  brake  on  a 
wheel,  checking  the  activity  of  the  automatic  beat.  These 
fibers  are  known  as  inhibitory  fibers,  and  their  action  is  a 
case  of  inhibition. 

24.  Inhibition.  —  In  the  examples   of   nervous   action 
which  we  have  thus  far  studied,  the  nervous  impulse  has 
uniformly  thrown  some  cell  into  activity.    The  stimulation 
of  muscle  fibers  to  contract,  of  gland  cells  to  secrete,  and 
of  nerve  cells  in  the  execution  of  reflexes  will  be  readily 
recalled.    To  this  same  class  of  nervous  actions  must  now 
be  added  that  of  the  augmentor  nerves  of  the  heart,  for 
they  excite  the  heart  to  greater  activity. 

In  the  inhibitory  nerves,  on  the  other  hand,  the  nervous 
impulse  produces  exactly  the  opposite  result.  Instead 
of  setting  organs  to  work  or  stimulating  .them  to  more 
vigorous  action,  they  diminish  activity,  and  in  extreme 
cases  check  or  stop  it  altogether.  In  our  subsequent 
studies  we  shall  meet  with  many  examples  of  this  effect; 
but  we  may  say  at  once  that  inhibition  is  as  characteristic 
and  as  important  a  feature  of  the  nervous  system  as  is 
excitation  (see  p.  285). 

25.  The  Regulation  of  the  Outflow  from  the  Arterial 
Reservoir.    Arterial  Tone.  —  Wound  around  the  walls  of 
the   arterial  tubes,    especially   the   smaller  arteries   (arte- 
rioles)  which  deliver  blood  from  the  arterial  reservoir  to 
the  organs,  are  peculiar  muscle  fibers.    Their  contraction 
diminishes  the  size  and  bore  of  the  tube,  and  when  they 


THE  CIRCULATION  OF  THE  BLOOD         159 

relax,  the  tube  and  its  lumen  become  wider.  As  a  usual 
thing  these  smaller  arteries  are  kept  somewhere  midway 
between  extreme  constriction  and  extreme  dilation  by  the 
sustained  moderate  contraction  of  their  muscle  fibers.  On 
a  day  of  moderate  temperature,  for  example,  the  arterioles 
of  the  skin  are  moderately  narrowed  by  this  action  of  their 
muscle  fibers.  During  colder  weather  these  fibers  contract 
more  than  .usual,  and  so  lessen  the  size  of  the  tube,  while 
during  warm  weather  they  relax  somewhat  and  widen  it ; 
but  ordinarily  they  are  never  contracted  to  their  utmost, 
nor  are  they  often  completely  relaxed. 

This  condition  of  sustained  activity  of  the  arterial  mus- 
cles is  known  as  arterial  tone,  and  in  general  any  sustained 
activity  of  a  living  cell  is  spoken  of  as  tonic  activity,  or  tone. 
Since,  as  we  have  seen,  the  total  quantity  of  blood  in  the 
body  is  not  enough  to  fill  completely  and  distend  all  the 
blood  vessels  when  they  are  widened  to  their  utmost,  it 
follows  that  the  maintenance  of  arterial  tone  is  essential 
to  that  overfilling  of  the  great  arteries  which  supplies  the 
driving  force  for  the  flow  of  blood  through  the  organs.  If 
every  arteriole  were  to  lose  its  tone,  blood  would  flow  out 
of  the  reservoir  more  rapidly  than  the  heart  could  possibly 
pump  it  in  ;  we  should  have  somewhat  the  same  con- 
dition of  affairs  as  if,  in  our  artificial  model  (p.  142),  the 
small  nozzle  which  affords  resistance  to  the  outflow  were 
removed.  Arterial  pressure  would  fall  and,  the  driving 
force  being  thus  removed,  the  blood  would  remain  at  rest 
in  the  capillaries  and  veins  of  the  organs ;  the  circulation 
would  cease  because  blood  would  not  return  to  the  heart 
to  be  pumped.  The  maintenance  of  arterial  tone  is  con- 
sequently no  less  essential  to  the  circulation  than  is  the 
beat  of  the  heart  itself. 

26.  Vasoconstrictor  Nerves.  --  The  muscle  fibers  of  the 
arteries  receive  nerves  which  stimulate  them  to  contract, 
for  if  these  nerves  are  cut,  the  arteries  lose  their  tone 


160  THE  HUMAN  MECHANISM 

(dilate).  We  conclude,  therefore,  that  the  ordinary  main- 
tenance of  arterial  tone  is,  in  part  at  least,  a  function  of 
the  nervous  system.  The  muscle  fibers  of  the  arteries,  in 
other  words,  remain  in  tonic  activity  because  the  neurones 
which  supply  them  with  nerve  fibers  are  in  tonic  activity ; 
and  we  can  understand  how  general  arterial  tone  may  be 
increased  or  decreased  by  the  condition  of  the  central  nerv- 
ous system,  by  reflexes,  by  the  nervous  "shock  "  of  surgical 
operations,  etc. 

Neurones  which  maintain  the  proper  amount  of  arterial 
tone  are  known  as  vasoconstrictor  neurones.  They  obvi- 
ously do  for  the  muscles  of  the  arteries  what  the  motor 
nerves  do  for  the  skeletal  muscles,  and  the  augmentors  do 
for  the  heart. 

27.  Vasodilator    Nerves.  —  Arteries,   however,   receive 
a  second  set  of  nerves,  which  have  exactly  the  opposite 
function,  i.e.  to  make   their  muscle  fibers  relax,  and  so 
lead  to  a  widening  or  dilation  of  the  artery.    These  nerves 
do  for  the  tonic  contraction  of  the  arteries  what  the  in- 
hibitory nerves  of  the  heart  do  for  the  heart  beat ;   they 
diminish  or  abolish  an.  existing  activity  and  thus  give  us 
our  second  example  of  inhibitory  nerves.    They  are  known 
as  the  vasodilators. 

The  vasodilators  are  not  regularly  in  tonic  activity  like 
the  vasoconstrictors.  They  are  called  into  action,  reflexly 
or  otherwise,  when  it  is  necessary  that  an  organ  receive 
more  blood  than  usual ;  at  other  times  the  vasoconstrictors 
are  free  to  exert  their  tonic  stimulation  and  so  regulate 
the  flow  of  blood  to  the  organ. 

28.  Importance  of  the  Nervous  System  to  the  Vascular 
Adjustments  of  Daily  Life.  —  It  is  impossible  within  the 
limits  of  the  present  work  to  enter  further  into  the  mode 
of  action  of  these   arterial   nerves,    or  to   discuss    other 
minor  influences  which  regulate  the  circulation  of  blood. 
Our  main  purpose  is  to  show  the  student  that  the  proper 


THE  CIRCULATION  OF  THE  BLOOD         161 

coordinated  working  of  the  nervous  system  is  as  important 
in  adapting  the  work  of  the  heart  and  blood  vessels  to  the 
hourly  needs  of  daily  life  as  it  is  in  producing  purposeful 
movements  of  the  skeletal  muscles.  Every  change  of  occu- 
pation and  activity,  every  change  of  surrounding  conditions 
of  temperature,  moisture,  wind,  etc.,  necessitates  some  spe- 
cial adjustment  of  the  vascular  system;  and  this  adjust- 
ment is  dependent  upon  the  same  sort  of  coordinating 
nervous  action  which  we  have  already  compared  with  the 
operations  of  a  large  army.  In  spite  of  the  fact  that  we 
are  for  the  most  part  unconscious  of  it,  it  is  none  the  less 
a  part  of  our  daily  life;  and  the  fatigue  induced  within  the 
mechanism  of  vasoconstrictor  and  vasodilator  neurones  by 
their  continued  activity  probably  contributes  a  large  share 
to  that  general  bodily  fatigue  which  leads  us  to  seek  recup- 
eration in  rest  and  sleep. 

The  apparatus,  the  operation,  and  the  regulation  of  the 
flow  of  blood  and  lymph  afford  an  excellent  illustration  of 
the  fact  that  the  human  body,  at  least  in  this  particular,  is 
a  complex  machine.  But  while  we  of  to-day  look  upon  it 
with  somewhat  less  of  awe  than  did  our  ancestors,  and 
while  there  is  for  us  less  of  mystery  and  more  of  mechan- 
ism in  it,  we  gain,  on  the  other  hand,  a  wholly  new  revela- 
tion of  its  intricacy,  and  a  fresh  sense  of  its  marvelous 
delicacy,  beauty,  and  perfection  of  adjustment.  The  mere 
fact  that  every  one  of  us  carries  in  his  bosom  a  powerful 
double  force-pump  of  remarkable  design,  original  construc- 
tion, and  extraordinary  power,  capable  in  many  instances 
of  successful  and  unremitting  service  for  more  than  three 
quarters  of  a  century,  should  be,  in  itself  alone,  enough  to 
excite  admiration  and  respect  for  the  entire  mechanism  of 
which  it  is  only  one  part,  and  to  awaken  within  us  a  desire 
to  use  that  mechanism  "  as  not  abusing  it." 


CHAPTER  X 
RESPIRATION 

1.  Breathing  is  not  the  Fundamental  Act  of  Respiration. 
—  We  have  found  in  studying  the  chemical  changes  which 
underlie  cellular  activity  (Chapter  IV)  that  muscle  fibers 
and  gland  cells  and,  we  may  now  add,  nerve  cells,  take  in 
oxygen  and  give  out  carbon  dioxide.    This  cell  breathing 
is  the  essential  act  of  respiration,  for  respiration  is  only 
another  name  for  the  oxidative  processes  of  the  living  body. 
Respiration  of  this  kind  (and  of  this  kind  only)  is  universal 
among  living  things.    The  one-celled  animal,  for  example, 
takes  its  oxygen  directly  from  the  free  oxygen  of  the  water 
in  which  it  lives,  and  discharges  its  carbon  dioxide  into  the 
same  surrounding  medium.    Every  one  of  the  thousands  of 
cells  of  which  the  human  body  is  composed  repeats  this 
same  process,  taking  its  oxygen  from,  and  discharging  its 
carbon  dioxide  into  its  surrounding  medium,  —  in  this  case 
the  lymph.  The  breathing  movements,  which  renew  the  air 
in  the  lungs,  and  the"  circulation  of  blood,  which  affords 
the  channel  of  communication  between  the  lungs  and  the 
tissues,  are  merely  accessory  mechanisms  rendered  neces- 
sary by  the  distance  of  the  cells  and  the  lymph  from  the 
surface  of  the  body.     Their  principal  function  is  to  keep 
the  lymph  supplied  with  oxygen  and  to  remove  from  it 
the  carbon  dioxide.    In  other  words,  breathing,  though 
ministering  to  respiration,   is  not  respiration  itself. 

2.  The  Quantity  of  Oxygen  and  of  Carbon  Dioxide  in  the 
Lymph  Surrounding  the  Cells  of  the  Body.  —  The  cell  is  the 
true  seat  of  oxidation.    Within  its  imperfectly  understood 

162 


KESPIKATION  163 

mechanism  are  found  the  conditions  which  lead  to  the 
union,  direct  or  indirect,  of  oxygen  with  the  proteids,  the 
carbohydrates,  and  the  fats  of  the  food. 

The  cell  draws  oxygen  from  the  surrounding  lymph  very 
much  as  a  burning  match  draws  oxygen  from  the  surround- 
ing air.  Consequently  the  amount  of  oxygen  dissolved  in 
the  lymph  is  generally  comparatively  small,  and  would  be 
removed  altogether  were  it  not  constantly  renewed  from 
the  blood. 

For  similar  reasons  the  lymph  must  be  relatively  rich 
in  carbon  dioxide,  since  it  is  this  fluid  which  directly 
receives  the  gas  (in  solution)  from  its  source  of  manufac- 
ture, the  working  cell.1 

3.  The  Quantity  of  Oxygen  and  of  Carbon  Dioxide  in  the 
Plasma  of  Arterial  Blood.  —  It  is  through  the  lungs  that  the 
body  as  a  whole  receives  its  oxygen  and  discharges  its 
excess  of  carbon  dioxide.  Consequently  arterial  blood 
contains  more  oxygen  and  less  carbon  dioxide  than  venous 
blood.  The  actual  figures  are  as  follows : 

Oxygen        Carbon  Dioxide    Nitrogen 

100  cc.  of  arterial  blood  contain         20  cc.  38  cc.  1-2  cc. 

100  cc.  of  venous  blood  contain      8-12  cc.         45-50  cc.  1-2  cc. 

These  figures  apply  to  the  whole  blood,  i.e.  to  plasma 
and  corpuscles  ;  but  what  is  true  of  the  whole  blood  is  true 
in  a  general  way  also  of  the  circulating  plasma,  which 
consequently  enters  the  capillaries2  relatively  rich  in  oxygen 
and  poor  in  carbon  dioxide,  thus  presenting  exactly  the 

1  The  gases  oxygen  and  carbon  dioxide  are,  of  course,  dissolved  in  the 
liquid  lymph  and  blood  plasma.     A  liquid  exposed  to  a  gas  absorbs  or 
dissolves  the  gas.    Thus,  100  cc.  of  water  when  exposed  to  atmospheric 
air  at  0°  C.  dissolves  4  cc.  of  oxygen  and  2  cc.  of  nitrogen. 

2  The  total  time  consumed  by  the  blood  in  passing  from  the  capillaries 
of  the  lungs  through  the  heart  to  those  or!  the  rest  of  the  body  seldom 
exceeds  five  or  six  seconds.    Hence  the  amount  of  the  gases  in  the  blood 
entering  the  capillaries,  for  example,  of  a  muscle  is  practically  the  same  as 
in  the  blood  leaving  the  lungs. 


164 


THE  HUMAN  MECHANISM 


reverse  composition,  in  respect  to  these  gases,  of  that  found 
in  the  lymph  surrounding  the  living  cells. 

4.  The  Passage  of  Oxygen  and  of  Carbon  Dioxide  be- 
tween the  Lymph  and  the  Blood  Plasma.  —  In  the  capillary 
regions  of  all  parts  of  the  body  except  the  lungs  we  have 
two  fluids,  the  lymph  and  the  blood  plasma,  containing 

very  different  amounts' 
of  oxygen  and  carbon 
dioxide,  and  separated 
from  each  other  by  the 
exceedingly  thin  mem- 
brane of  the  capillary 
wall.  Under  such  con- 
ditions both  gases  will 
tend  to  equalize,  and 
each  gas  will  pass 
through  the  membrane 
from  that  liquid  in 

FIG.  71.  The  passage  of  oxygen  and  car-  which  it  is  more  abun- 
bon  dioxide  between  the  blood  and  the  dant  to  that  in  which  ifc 
lymph  in  the  tissues  ,  . 

is  less  abundant ;  that 

is  to  say,  the  oxygen  will  pass  from  the  blood  plasma  in 
which  it  abounds  to  the  lymph  in  which  it  is  scarce ;  and 
the  carbon  dioxide  in  the  other  direction,  from  the  lymph 
to  the  blood  plasma  (see  Fig.  71).  Hence  the  blood  enters 
the  veins  richer  in  carbon  dioxide  and  poorer  in  oxygen 
than  it  left  the  arteries. 

5.  The  Red  Corpuscle  as  a  Carrier  of  Oxygen. — The  blood 
plasma  under  the  conditions  of  temperature  and  pressure 
to  which  it  is  exposed  can  hold  in  mere  solution  only  a 
small  amount  of  oxygen,  far  too  little  to  meet  satisfactorily 
the  demands  of  the  resting  tissues,  and  utterly  inadequate 
for  the  much  greater  needs  of  the  working  tissues.    This 
difficulty  is  met  and  the  oxygen-carrying  capacity  of  the 
blood  vastly  increased  by  the  peculiar  properties  of  the 


RESPIRATION  165 

coloring  matter,  or  pigment,  of  the  red  corpuscles.  This  sub- 
stance, known  as  hemoglobin,  readily  forms  with  oxygen 
a  compound  (oxyhemoglobin)  whenever  the  amount  of  oxygen 
is  high  in  the  medium  surrounding  it;  if,  however,  much 
oxygen  is  removed  from  its  surrounding  medium,  the  oxy- 
hemoglobin  breaks  up  or  dissociates  into  hemoglobin  and 
free  oxygen.  Applying  this  to  the  conditions  in  the  capil- 
laries, we  find  that  100  cc.  of  arterial  blood  contain  less 
than  1  cc.  of  free  oxygen  in  the  plasma,  but  about  19  cc. 
of  oxygen  combined  in  the  oxyhemoglobin  of  the  red  cor- 
puscles. When  the  blood  enters  the  capillaries  of  living 
tissues,  oxygen  passes,  as  we  have  seen,  from  the  plasma 
into  the  lymph,  so  that  the  oxygen  content  of  the  plasma 
is  reduced.  When  this  reduction  goes  below  a  point 
which  is  quickly  reached,  dissociation  of  the  oxyhemoglo- 
bin occurs,  and  the  oxygen  thus  set  free  in  the  plasma  is 
drawn  away  by  the  lymph,  from  which  it  is  in  turn  drawn 
by  the  cell,  the  real  seat  of  oxidation. 

It  is  important  that  the  student  should  clearly  un- 
derstand that  the  cell  is  the  master  of  the  respiratory 
processes,  not  their  servant, —  uWe  may  lead  a  horse  to 
water  or  fetch  water  to  a  horse,  but  we  cannot  make  him 
drink," —  and  that  the  cause  of  this  movement  of  oxygen 
step  by  step  from  the  corpuscle  to  the  plasma,  thence  to  the 
lymph,  and  thence  to  the  cell,  is  the  utilization  of  oxygen 
by  the  cell  for  its  life  work,  not  the  presence  of  oxygen 
in  the  blood ;  and  the  chemical  activities  within  the  cell, 
not  the  amount  of  oxygen  brought  by  the  blood,  pri- 
marily determine  how  much  oxygen  shall  pass  to  it  from 
the  blood.  This  fundamental  truth  will  be  emphasized  as 
we  proceed. 

6.  Gaseous  Exchanges  of  the  Blood  in  the  Tissues. —  How 

,much  oxygen  is  taken  from  the  blood  by  the  tissues  and 

how  much  carbon  dioxide  is  received?    This  question  might 

be  answered  for  the  body  as  a  whole  by  comparing  the 


166  THE  HUMAN  MECHANISM 

amounts  of  these  gases  in  the  blood  of  the  large  arteries 
and  in  that  of  the  large  veins. 

Oxygen      Carbon  Dioxide      Nitrogen 

100  cc.  of  arterial  blood  contain          20  cc.  38  cc.  1-2  cc. 

100  cc.  of  venous  blood  contain      8-12  cc.        45-50  cc.  1-2  cc. 

It  appears  from  these  figures  that  there  is  always  a  very 
considerable  amount  of  carbon  dioxide  in  arterial  blood,  and 
a  very  considerable  amount  of  oxygen  in  venous  blood;  in 
other  words,  that  the  tissues  do  not  remove  all  the  oxygen 
nor  do  the  lungs  remove  all  the  carbon  dioxide  from  the 
circulating  blood.  What  the  processes  of  respiration  ac- 
complish is  to  keep  the  gaseous  content  of  the  arterial 
blood  nearly  constant  at  the  figures  given  above;  and  except 
in  the  case  of  suffocation  (asphyxia),  the  gaseous  content 
of  arterial  blood  does  not  vary  widely  from  that  given. 
It  was  formerly  assumed  that  in  muscular  exercise  the 
increased  consumption  of  oxygen  and  production  of  carbon 
dioxide  by  the  working  muscles  made  the  arterial  blood 
more  venous,  i.  e.  poorer  in  oxygen,  or  richer  in  carbon 
dioxide,  or  both;  but  it  is  now  known  that  the  arterial 
blood  during  exercise  contains  about  the  same  amounts  of 
oxygen  and  carbon  dioxide  as  during  rest. 

Not  only  is  this  true,  but  it  has  also  been  proved  that 
the  amount  of  oxygen  given  above  is  almost  all  that  can 
be  taken  up  by  the  blood,  when  it  flows  through  the  lungs 
under  the  atmospheric  pressures  to  which  we  are  ordinarily 
subjected.  An  elementary  text-book  is  not  the  place  for 
the  details  of  this  subject ;  but  even  the  beginner  should 
understand  that  neither  by  deeper  nor  by  more  rapid 
breathing,  nor  by  any  change  in  the  character  of  breathing, 
is  it  possible  to  increase  appreciably  the  amount  of  oxygen 
absorbed  by  the  same  volume  of  blood  flowing  through  the 
lungs.  The  assertion,  too  frequently  heard,  that  some 
special  form  of  breathing  movements  leads  to  more  efficient 


EESPIEATION  167 

oxidation  of  the  body  wastes  betrays  lamentable  ignorance 
of  this  fundamental  fact  of  physiology.  This,  however,  is 
not  denying  that  one  type  of  breathing  movement  may  still, 
for  other  reasons,  be  preferable  to  another,  nor  affirming 
that  deepened  breathing  may  not  sometimes  be  desirable. 
Breathing  movements  accomplish  other  things  than  oxygen- 
ation  of  the  blood,  and  we  may  now  proceed  to  study  their 
physiology. 

7.  Structure  of  the  Lungs.  —  In  Chapter  II  the  anatom- 
ical relations  of  the  air  passages  (trachea,  bronchi,  etc.)  and 
lungs  have  been  described.  The  student 
at  this  point  should  consult  especially 
Fig.  5  (p.  13)  in  order  to  obtain  a  clear 
idea  of  the  structure  of  the  lungs.  The 
bronchi  which  enter  the  lungs  branch, 
much  as  the  ducts  of  a  gland,  and  their 
ultimate  branches  end  in  the  alveoli, 
which,  like  those  of  a  gland,  consist  of  a 

single  layer  of  cells,  but  in  this  case  of  FlG-  72-  Two  adJa~ 
,7  •        n    .          7       77        T\'       no     i_  cent  alveoli  of  the 

very  thin,  flattened  cells,    .rig.  12  snows       ^    a 

two  of  these  alveoli  dissected,  and  Fig.  73  Showjg  the  air  cells 
a  section  taken  lengthwise  through  the 
same.  Connective  tissue  binds  together  the  alveoli  and 
bronchial  tubes,  thus  forming  the  lobes  of  the  lungs.  In 
this  connective  tissue  —  and  hence  between  the  alveoli  — 
are  the  larger  blood  vessels,  branches  of  the  pulmonary 
artery  and  pulmonary  veins.  The  arterioles  supply  an 
exceedingly  close  network  of  capillaries  (Fig.  139),  which 
are  in  direct  contact  with  the  lining  cells  of  the  alveolus, 
so  that  the  blood  in  these  capillaries  is  separated  from  the 
air  in  the  alveolus  only  by  the  thin  capillary  wall  and  the 
equally  thin  layer  of  flattened  alveolar  cells.  Under  these 
circumstances  the  exchange  of  oxygen  and  carbon  dioxide 
takes  place  readily  between  the  air  in  the  lungs  and  the 
blood  in  the  capillaries.  Finally  the  absorbing  surface  of 


168  THE  HUMAN  MECHANISM 

the  alveolar  wall  is  greatly  increased  by  being  arranged  in 
the  form  of  pits  or  air  cells,1  as  shown  in  Figs.  5,  72,  73, 
and  139. 

8.  Purpose  of  Breathing  Movements.  —  As  the  blood  is 
constantly  giving  up  carbon  dioxide  to,  and  taking  oxygen 


FIG.  73.  Diagram  of  a  longitudinal  section  of  two  alveoli  with  their  com- 
mon bronchiole  and  showing,  in  black,  the  larger  blood  vessels  in 
the  connective  tissue 
The  capillary  network  belonging  to  these  vessels  is  shown  in  Fig.  139 

from  the  air  of  the  lungs,  this  air  would  soon  cease  to  be  of 
use  in  purifying  the  blood,  were  it  not  for  the  breathing 
movements,  whose  function  is  to  replace  the  vitiated  ail- 
within  the  lung  with  pure  air  from  without.  Breathing 
is,  accordingly,  an  act  of  ventilation  of  the  lungs,  and  it  is 
the  stoppage  of  this  ventilation  which  produces  suffocation 
or  asphyxia. 

1  The  word  "  cell "  is  here  used  to  represent  a  hollow  space,  and  not 
with  its  usual  histological  meaning. 


BESPIBATIOJST 


169 


9.  Mechanics  of  the  Breathing  Movements.  —  A  knowl- 
edge of  the  mechanism  of  the  breathing  movements  is  of 
much  practical  importance,  especially  in  hygiene,  and  may 
be  understood  without  great  difficulty  by  the  study  of  the 
model  shown  in  Fig.  74.  The  trachea  and  the  bronchi  are 
represented  by  the  glass  tube,  and  the  lungs  by  an  elastic 
bag,  L,  at  the  end  of  the  tube.  The 
lungs  lie  in  the  large  air-tight  thorax 
which  incloses  the  pleural  or  thoracic 
cavity  (p.  10).  This  thoracic  wall  is 
represented  in  the  model  by  a  glass 
bell  jar  closed  beneath  by  a  sheet  of 
thick  rubber,  D.  The  cavity  of  the 
bell  jar  represents  the  pleural  cavity, 
and  the  rubber  represents  the  dia- 
phragm (see  Fig.  134).  The  condition 
of  the  lung  in  the  pleural  cavity  may 
be  still  further  imitated  in  the  model 
by  partially  exhausting  the  air  in  the 
bell  jar.  The  distensible  and  elastic 
rubber  bags  now  inflate,  because  the  atmospheric  pressure 
within  them  is  greater  than  the  pleural  pressure  without. 
We  shall  not  stop  to  explain  why  pleural  pressure  is  less 
than  that  of  the  atmosphere;  but  knowing  the  fact,  it  is 
easy  to  understand  why  the  lungs  should  be  distended,  so 
as  virtually  to  fill  those  portions  of  the  thoracic  cavity  not 
occupied  by  the  heart,  great  blood  vessels,  and  other  organs.1 

If  now  the  size  of  the  pleural  cavity  of  our  model  be 
enlarged  by  pulling  downwards  the  sheet  of  rubber  at  the 
bottom,  the  pressure  will  be  still  further  lowered  in  the 
cavity  and  the  lungs  will  expand.  At  the  same  time  air 
is  sucked  in  through  the  glass  trachea  and  mixes  with 

1  The  student  is  again  warned  against  supposing  that  the  pleural  cavity 
is  a  large  space  filled  with  air ;  in  this  respect  the  model  is  misleading, 
since  the  lungs  and  other  organs  completely  fill  the  thoracic  cavity. 


FIG.  74.  Model  of  the 
action  of  the  thoracic 
walls  and  lungs  in 
respiration.  See  Sec- 
tion 9 


170  THE  HUMAN  MECHANISM 

that  in  the  model  lungs.  When  the  pull  is  released,  the 
pleural  pressure  rises,  the  lungs  grow  smaller,  and  air  is 
forced  out.  In  this  way  the  mechanism  of  the  ventilation 
of  the  lungs  may  be  imitated  in  essential  particulars. 

In  life  the  pleural  cavity  is  enlarged  during  inspiration 
by  the  contraction  of  the  diaphragm  and  the  elevation  of 
the  ribs.  Both  of  these  are  movements  effected  by  the 
action  of  skeletal  muscles.  The  understanding  of  the  ele- 
vation of  the  ribs  need  give  no  difficulty;  muscles,  some 
of  which  are  shown  in  Fig.  12,  pull  upwards  on  the  ribs; 
and  the  attachment  of  the  ribs  to  the  vertebral  column 
and  the  breastbone  (sternum)  is  such  that  when  they  are 
raised  the  diameter  of  the  thorax  is  increased  dorso-ven- 
trally  and  from  side  to  side.  The  diaphragm,  on  the  other 
hand,  is  a  kind  of  circular  muscle  with  a  central  fibrous  or 
tendinous  portion  from  which  the  bundles  of  muscle  fibers 
radiate  outwards  to  its  edges.  Any  shortening  of  these 
fibers  evidently  diminishes  the  diameter  of  the  diaphragm ; 
and  because  of  its  form  (that  of  a  dome  directed  upwards 
into  the  thoracic  cavity),  contraction  of  this  muscle  must 
increase  the  size  of  the  lower  thorax.1 

There  are  three  typical  modes  of  breathing:  (1)  The  pre- 
dominantly costal  or  "  rib  "  breathing.  Here  the  diaphragm 
is  but  little  used.  It  is  the  type  characteristic  of  those  who 
impede  movements  of  the  lower  ribs  and  abdomen  with 
constricting  clothing,  such  as  tight  corsets.  (2)  The  pre- 
dominantly abdominal.  Here  the  ribs  are  little  used  while 

1  The  action  of  the  diaphragm  is  often  described  as  increasing  the 
antero-posterior  (head  to  foot)  dimension  of  the  thorax;  but  this  can 
happen  only  when  the  diaphragm  is  free  to  descend,  and  it  can  descend 
only  when,  by  displacing  downwards  the  contents  of  the  abdominal 
cavity,  it  causes  the  well-known  respiratory  movements  of  the  abdominal 
walls.  These  "abdominal  movements"  may,  however,  be  prevented  by 
the  simultaneous  contraction  of  the  abdominal  muscles.  In  this  case  the 
diaphragm  cannot  descend,  and  its  contraction  can  only  raise  the  lower 
ribs  to  which  it  is  attached.  The  mechanism  in  these  two  methods  of 
using  the  diaphragm  is  clear  from  Fig.  75. 


EESPIEATION 


171 


the  diaphragm  does  most  of  the  work,  the  abdominal  mus- 
cles being  relaxed  so  that  the  belly-wall  has  its  maximum 
of  movement.  This  type  of  breathing  involves  great  relax- 
ation of  tone  of  the  abdominal  muscles,  which  is  a  serious 
disadvantage.  (3)  The  lateral  costal.  Here  the  abdominal 
muscles  act  at  the  same  time  as  the  ribs  and  diaphragm. 


FIG.  75.  Action  of  the  diaphragm  in  "  abdominal "  and  in 
"  lateral  costal  "  breathing 

Solid  lines  represent  position  of  body  wall,  diaphragm,  and  ribs  during  ex- 
piration; dotted  lines,  the  same  during  inspiration.  The  left-hand  figure 
represents  abdominal  breathing,  the  diaphragm  becoming  more  convex, 
displacing  downward  the  abdominal  viscera,  and  forcing  outward  the  ab- 
dominal body  wall.  In  the  lateral  costal  type  the  diaphragm  raises  the 
lower  ribs  and  the  abdominal  walls  may  actually  move  inward,  owing  to 
the  contraction  of  their  muscles 

This  form  of  breathing  produces  the  highest  pressures 
on  the  contents  of  the  abdominal  cavity,  and  maintains  the 
tone  of  the  abdominal  walls,  without  diminishing  the  effi- 
ciency of  the  oxygenation  of  the  blood.  It  also  forces' the 
use  of  the  upper  ribs  to  a  much  greater  extent  than  does 
the  predominantly  abdominal  type  of  breathing  (Fig.  75). 


172  THE  HUMAN  MECHANISM 

It  is  seldom  that  one  or  another  of  these  types  is  used 
in  its  entirety,  and  the  advantages  of  one  form  over  another 
are  often  greatly  exaggerated.  The  following  statements 
may,  however,  be  taken  as  summing  up  the  essential 
practical  points. 

1.  The  breathing  movements  should  be  such  as  to  use  all 
portions  of  the  lungs.    In  the  abdominal  type  there  is  little 
or  no  movement  of  the  upper  thorax.    The  result  is  that 
the  apical  or  upper  lobes  of  the  lungs  do  not  share  in  the 
enlargement  and  contraction  of  the  lungs;  they  are  poorly 
ventilated,  their  lymph  current  —  which  largely  depends 
upon  these  movements  —  becomes  sluggish,  arid  because  of 
these  unfavorable  physiological  conditions  there  is  greater 
liability  to  disease.  More  than  sixty  per  cent  (some  observers 
claim  as  many  as  eighty  per  cent)  of  the  beginnings  of  the 
lung  ravages  of  pulmonary  consumption  are  found  in  this 
portion  of  the  lung ;  and  this  is  believed  to  be  due  to  the 
lack  of  movement  which  results  from  the  failure  to  use 
the  upper  thorax. 

2.  Actual  study  of  the  breathing  movements  in  people 
who  have  not  worn  constricting  clothing  indicates  that  the 
enlargement  of   the  thorax  in  inspiration   is  effected  by 
the  approximately  equal  action  of  the  diaphragm  and  of 
the  muscles  which  elevate  the  ribs. 

3.  The  abdominal  muscles  should  to  some  extent  con- 
tract with  the  diaphragm.    This  is  especially  important  in 
those  whose  occupation  is  more  or  less  sedentary,  as  it  is 
the  most  convenient  means  of  giving  to  these  muscles  the 
use  which  is  essential  to  the  maintenance  of  their  strength, 
and  the  consequent  prevention  of  that  loss  of  tone  which 
takes  away  from  the  organs  of  the  abdominal  cavity  one 
of  their  chief  supports.    (Consult  Part  II,  Chapter  XVIII.) 

4.  There  are  good  reasons  for  thinking  that  it  is  impor- 
tant to  develop  properly  the  muscles  of  the  upper  thorax 
and  especially  those  which  lie  in  the  triangle  between  the 


EESPIRATION  173 

root  of  the  neck,  the  collar  bone,  and  the  shoulder  blade. 
When  these  muscles  are  not  developed,  especially  in  thin 
people,  the  wall  of  the  thorax  in  this  region  sinks  inward 
during  inspiration ;  under  these  circumstances  this  portion 
of  the  thorax  is  not  enlarged  during  inspiration,  the  apical 
lobes  no  longer  share  in  the  expansions  and  contractions 
of  the  lungs,  and  imperfect  ventilation  of  this  part  of  the 
lung  results. 

10.  Secondary  Effects  of  the  Breathing  Movements.  - 
The  student  will  now  be  better  able  to  understand  the  part 
taken  by  the  breathing  movements  in  facilitating  the  return 
of  blood  and  lymph  to  the  heart.    The  enlargement  of  the 
thorax  during  inspiration  sucks  blood  and  lymph  in  toward 
the  great  veins  by  the  same  process  that  it  sucks  air  into 
the  lungs.1    Especially  in  the  case  of  the  lymph  flow  is 
this  a  most  important  factor.    Moreover,  in  the  lymphatics 
of  the  lungs,  situated  as  they  are  entirely  within  the  thorax, 
the  movements  of  the  lungs  during  respiration  pump  the 
lymph  onwards    and  are    of   special   importance  in   this 
respect.    Much   of  the    invigorating    effect  of   muscular 
exercise,  popularly  ascribed  to  better  oxygenation  of  the 
blood   and  tissues,  is  really  attributable    to  the   greatly 
improved  lymph  flow  from  all  organs  which  results  from 
the  deepened  respiration  in  muscular  activity. 

11.  The  Nervous  System  as  a  Factor  in  Respiration.— 
The  muscles  of  the  diaphragm  and  those  of  the  ribs,  like 
the  biceps  and  other  muscles  which  act  upon  the  skeleton, 
are  stimulated  to  contraction  by  nervous  impulses  from  the 
brain  and  spinal  cord.    Every  movement  of  respiration  is 
called  forth  and  regulated,  in  accordance  with  the  needs 
of  the  body  at  the  time,  by  the  coordinated  action  of  a 
number  of  nerve  cells.    Those  which  are  most  intimately 

1  The  model  of  the  thorax  (p.  169)  may  be  made  to  show  this  action 
in  the  veins  and  lymphatics  by  the  addition  of  a  second  bag  to  represent 
the  blood  vessels. 


174  THE  HUMAN  MECHANISM 

concerned  with  respiration  are  found  in  different  parts  of  the 
central  nervous  system,  from  the  lower  portion  of  the  brain 
to  the  end  of  the  first  half  of  the  spinal  cord,  inclusive; 
and  there  is  good  reason  for  thinking  that  nerve  cells  in 
the  lower  portion  of  the  brain  send  out  the  stimuli  to  those 
of  the  cord,  and  through  them  excite  the  muscles  to  contract. 
Their  precise  mode  of  stimulation,  however,  is  too  complex 
for  description  in  an  elementary  work  on  physiology. 

12.  The  Circulation  as  an  Essential  Part  of  the  Mechanism 
of  Respiration.  —  The  consumption  of  oxygen  and  the  pro- 
duction of  carbon  dioxide  thus  involve  an  interchange  of 
these  gases  between  the  blood  and  the  tissues  (internal 
respiration)  on  the  one  hand,  and  between  the  blood  and 
the  air  in  the  lungs  (external  respiration)  on  the  other. 
But  to  carry  out  these  gaseous  exchanges  a  third  factor  is 
obviously  necessary,  namely,  a  means  of  communication 
between  the  two,  so  that  the  oxygen  absorbed  in  the  lungs 
may  be  carried  to  the  tissues,  and  the  carbon  dioxide  pro- 
duced in  the  tissues  be  carried  back  to  the  lungs.  This 
communication  is  provided,  as  has  been  shown  in  earlier 
chapters,  by  the  circulation,  which  thus  becomes  an  essen- 
tial part  of  the  respiratory  mechanism. 

We  have  already  seen  that  under  the  most  varying  con- 
ditions 100  cc.  of  arterial  blood  always  contain  approxi- 
mately 20  cc.  of  oxygen  and  38  cc.  of  carbon  dioxide ;  and 
that  this  is  practically  all  the  oxygen  this  amount  of  blood 
can  hold.  From  this  it  follows  that  so  long  as  the  amount 
of  blood  pumped  by  the  heart  in  a  given  time  remains 
constant,  no  more  oxygen  will  be  carried  to  the  tissues, 
even  if  we  breathe  more  deeply.  In  other  words,  increased 
ventilation  of  the  lungs  without  any  accompanying  increase  in 
the  rate  and  force  of  the  heart  beat  will  not  supply  more 
oxygen  to  the  tissues.  The  beat  of  the  heart  is  as  impor- 
tant to  proper  tissue  respiration  as  are  the  deepened  breath- 
ing movements  ;  and  we  find  accordingly  that  these  two 


RESPIRATION  175 

events  are  closely  coordinated.  Greatly  increased  tissue 
respiration  invariably  carries  along  with  it  increased  work 
on  the  part  of  the  heart. 

A  large  number  of  measurements  of  the  respiratory 
exchanges 1  under  different  conditions  and  activities  of 
our  life  has  shown  that  these  are  increased  by  the  taking 
of  food,  by  exposure  to  cold,  by  awaking  from  sleep,  and, 
above  all,  by  muscular  activity.  The  last  three  really  be- 
long under  the  one  head  of  muscular  activity ;  for  when 
we  are  exposed  to  cold  we  move  about  more  actively,  or 
if  we  do  not,  we  "  shiver  " ;  and  the  same  thing  is  true  of 
awakening  from  sleep.  The  taking  of  food  does  not  lead 
to  any  remarkable  increase  in  gaseous  exchange,  except 
when  one  passes  from  a  period  of  prolonged  fasting  to 
one  of  feeding ;  but  this  event  is  so  abnormal  that  it  may 
be  neglected.  Consequently  we  have  left  the  one  condition 
of  muscular  activity  as  the  important  means  of  increasing 
tissue  respiration. 

And  this  increase  is  at  times  very  great.  Even  the  mus- 
cular activity  necessary  to  maintain  the  erect  position  in 
sitting  and  standing,  as  compared  with  the  complete  relax- 
ation of  sleep,  doubles  the  gaseous  exchange ;  gentle  exer- 
cise (a  walk  of  three  miles  an  hour)  more  than  doubles 
that  of  rest;  and  vigorous,  yet  by  no  means  excessive, 
exercise  will  increase  it  tenfold.  These  increases  mean 
corresponding,  though  not  absolutely  proportionate,  de- 
mands on  the  heart,  and  emphasize  the  importance  of 
keeping  that  organ  in  an  efficient  working  condition. 
Breathlessness,  for  example,  usually  indicates,  in  part  at 
least,  that  the  heart  fails  to  respond  properly  to  the 
demands  made  upon  it,  these  demands  being  greater  than 
it  can  meet  without  undue  fatigue ;  it  is  a  warning  that 
we  are  pushing  the  heart  too  hard,  a  warning  which  we 
will  do  well  to  heed.  Generally  it  is  also  a  warning  that  we 
1  i.e.  oxygen  absorbed  and  carbon  dioxide  discharged  in  a  given  time. 


176  THE  HUMAN  MECHANISM 

are  not  getting  sufficient  muscular  activity ;  the  heart  fails 
to  meet  the  emergency  of  some  unusual  exertion  because 
all  along  it  has  not  been  kept  in  proper  training ;  so  that 
while  we  should,  as  stated,  heed  the  warning  not  to  push 
the  heart  so  hard  for  the  time  being,  we  should  also  act 
upon  the  equally  important  warning  that  it  needs  practice 
or  training,  —  a  training  which  can  be  given  only  by  rea- 
sonable, regular,  muscular  activity. 

The  training  of  muscular  activity  is  therefore  not  only 
a  training  of  the  muscles  but  also  of  the  heart.  But  this 
is  not  all.  The  work  of  the  circulatory  and  respiratory  mechan- 
isms must  be  adjusted  or  coordinated,  the  one  to  the  other. 
When,  for  example,  the  deepened  breathing  movements 
accompanying  muscular  activity  rush  the  blood  back  more 
rapidly  to  the  heart  (p.  145),  it  becomes  necessary  for  the 
heart  to  adjust  the  character  of  its  beat  to  the  new  condi- 
tions ;  and  this  adjustment  is  the  work  of  the  nervous  sys- 
tem. Time  is,  however,  required  to  make  the  adjustment, 
so  that  it  is  wise  to  "  warm  up  "  gradually  to  more  vigor- 
ous work.  We  can  also  understand  how  by  physical  train- 
ing this  process  of  adjustment  comes  to  be  shortened ;  for 
we  have  not  only  trained  the  heart  by  giving  it  more  work 
to  do,  but  we  have  also  trained  those  portions  of  the  ner- 
vous system  which  regulate  its  beat. 


CHAPTER  XI 
EXCRETION 

We  have  seen  that  through  the  chemical  activity  of 
the  cells  of  the  body  waste  products  are  produced  from 
the  food,  and  this  largely  by  a  process  of  oxidation.  We 
have  also  learned  that  the  presence  of  undue  quanti- 
ties of  these  wastes  interferes  with  many  normal  activities 
of  the  cells.  In  the  organs  of  the  body,  as  in  the  houses 
of  a  city,  wastes  tend  to  accumulate,  and  both  for  health 
and  for  decency,  they  must  be  removed.  It  is  this  removal 
which  is  known  by  the  general  name  of  excretion. 

1.  The  Waste  Products  of  the  Human  Mechanism. — We 
may  first  consider  the  nature  of  the  more  important  wastes, 
and  then  the  means  by  which  they  are  eliminated  from  the 
body. 

The  oxidation  of  fats  takes  place  according  to  the  fol- 
lowing equation : 

C51H98O6  +  71  O2  =  51  CO2  +  46  H2O. 

In  other  words,  they  are  changed  to  carbon  dioxide  and 
water.    The  same  thing  is  true  of  the  carbohydrates: 

C6H1206  +  6  02  =  6  H20  +  6  CO2. 

In  the  case  of  the  proteids  and  such  allied  compounds 
as  gelatin  the  chemical  changes  are  more  complex  and 
the  products  of  the  change  more  numerous.  These  sub- 
stances, unlike  the  fats  and  carbohydrates,  contain  nitrogen 
in  addition  to  carbon  and  hydrogen,  and  their  waste  prod- 
ucts comprise  both  nitrogenous  and  non-nitrogenous  com- 
pounds. Of  the  former,  urea  (CON2H4)  occurs  in  largest 

177 


178  THE  HUMAN  MECHANISM 

quantity;  indeed,  it  constitutes  most  of  the  nitrogenous 
excretion  of  the  body;  other  nitrogenous  substances  are 
found  in  small  quantities,  among  which  may  be  mentioned 
uric  acid  (C5H4N4O3),  the  nearly  related  xanthin  bases, 
hippuric  acid,  and  creatinin.  The  non-nitrogenous  wastes 
of  proteids  are  chiefly  carbon  dioxide  and  water;  others 
that  we  need  not  mention  occur  in  small  quantities. 

The  main  excretions  of  the  body  are,  therefore,  carbon 
dioxide,  water,  and  urea,  together  with  small  amounts  of 
other  nitrogenous  and  non-nitrogenous  compounds.  In 
addition  to  these  the  body  is  constantly  excreting  inorganic 
salts,  sodium  chloride  being  the  largest  in  amount. 

2.  The  Organs  of  Excretion.  —  Excretion,  like  absorp- 
tion, must  obviously  take  place  either  through  the  outer 
surface  of  the  body,  —  the  skin,  —  or  else  in  organs  like 
the  lungs,  the  intestine,  or  the  kidneys,  which  directly  com- 
municate with  this  outer  surface  ;  and  we  actually  find 
that  waste  products  leave  the  blood  through  all  of  these 
channels.  An  organ  may,  however,  be  essential  to  the 
proper  removal  of  a  given  waste  product,  or  it  may  remove 
the  waste  product  only  incidentally  in  performing  its  essen- 
tial functions.  Thus  the  skin  removes  a  small  amount 
of  carbon  dioxide  from  the  body  merely  because  a  certain 
amount  of  this  gas  diffuses  from  the  blood  as  it  flows 
through  the  skin.  It  is  not  necessary  to  the  health  of  the 
body  that  the  skin  should  excrete  this  carbon  dioxide,  for 
the  lungs  are  quite  capable  of  doing  the  work,  and  would 
do  so  if  for  any  reason  such  excretion  through  the  skin 
were  prevented.  Without  the  lungs,  on  the  other  hand, 
the  carbon  dioxide  would  rapidly  accumulate  in  the  blood 
and  cause  death.  The  lungs  are  essential  to  the  removal 
of  this  waste  ;  the  skin  is  not.  Similarly,  the  perspiration 
contains  small  amounts  of  urea  and  other  wastes  which  are 
removed  in  large  quantities  by  the  kidneys.  It  is  not 
necessary  that  the  skin  should  remove  any  of  these,  for 


EXCBETION  179 

the  healthy  kidney  can  and  does,  when  necessary,  remove 
them.  They  appear  in  the  perspiration  because  they  are 
in  the  blood  from  which  the  perspiration  is  formed,  and 
because  the  cells  of  the  sweat  glands  allow  them  to 
pass  through,  just  as  the  skin  allows  the  passage  of  carbon 
dioxide. 

These  considerations  are  of  practical  importance  in  the 
hygiene  of  the  skin.  It  is  not  necessary  to  induce  perspi- 
ration merely  to  remove  waste  products  from  the  body. 
If  the  human  skin,  like  that  of  the  cat,  and  the  dog,  con- 
tained no  sweat  glands,  the  waste  products  would  be  as 
thoroughly  removed  as  they  are  now  ;  and  in  cold  weather, 
when  no  perspiration  is  being  secreted,  the  excretion  of 
waste  is  as  complete  as  when  in  warm  weather  perspiration 
is  abundantly  secreted. 

The  most  important  organs  of  excretion  are  the  lungs 
and  the  kidneys ;  next  to  these  should  be  placed  the 
intestine.  Each  of  these  is  essential  to  the  elimination  of 
one  or  more  of  the  waste  products  from  the  blood.  The 
skin,  on  the  other  hand,  as  just  shown,  is  only  incidentally 
an  organ  of  excretion.  The  chief  wastes  leaving  the  body 
by  each  of  these  channels  are  given  in  the  following  table, 
the  incidental  excretions  being  in  italics. 

Lungs  :  carbon  dioxide,  water. 

Kidneys :    urea,    uric    acid,    and   other   nitrogenous   com- 
pounds, salts,  water. 

Intestine  :  bile  pigments,  nitrogenous  compounds,  etc. 
Skin  :  urea,  etc.,  salts,  water. 

The  structure  and  action  of  the  lungs  and  intestine 
have  already  been  described  ;  so  that  we  have  left  for 
study  the  kidneys  and  the  skin. 

3.  Structure  of  the  Kidneys.  —  Each  kidney  is  a  bean- 
shaped  gland  whose  duct,  the  ureter,  runs  to  the  urinkry 
bladder.  As  the  ureter  enters  the  kidney  at  the  center  of 
the  depression  in  that  organ  it  expands  to  form  a  large 


180 


THE  HUMAN  MECHANISM 


Vena  Cava 
-  Ureter 


basin,  known  as  the  pelvis  of  the  ureter.  Into  this  basin 
open  the  hundreds  of  glandular  tubules  of  which  the  bulk 
of  the  kidney  is  composed.  Each  tubule,  like  the  alveolus 

and  ducts  of  the  gland  de- 
scribed in  Chapter  III,  con- 
sists of  a  single  layer  of  cells, 
which  separate  the  blood  and 
lymph  from  the  lumen  of  the 
tubule ;  and  the  formation  of 
urine  by  the  kidney  is  essen- 
tially an  act  of  secretion. 

4.  The  Secretion  of  Urine. 
-The  urine  is  being  con- 
stantly secreted  from  the 
blood,  at  one  time  more  rap- 
idly than  at  another,  but  un- 
der normal  conditions  never 
ceasing  altogether.  Passing 
down  the  tubules,  it  collects 
at  the  upper  portion  of  the 
ureter,  and  successive  peri- 

and  abdominal  aorta  and  vena    staltic  waves  carry  it  from  this 
cava  point  to  the  urinary  bladder, 

an  organ  with  muscular  walls,  in  which  the  urine  accumu- 
lates and  from  which  it  is  from  time  to  time  discharged. 
Secietion  by  the  kidney,  however,  presents  some  pecul- 
iarities which  make  it  unlike  the  secretion  of  the  gastric 
juice  or  the  saliva.  We  have  seen  that  the  secretion  of 
these  latter  juices  is  called  forth  by  nervous  impulses ;  but 
in  the  kidney  the  main  factor  which  determines  the  amount 
of  urine  secreted  is  the  amount  of  blood  flowing  through 
that  organ. 

I*b  has  been  shown  that  any  increase  of  blood-flow 
through  the  kidney  is  accompanied  by  an  increased  secre- 
tion of  urine,  and  vice  versa.  The  same  thing  is  seen  in 


EIG.  76.  Dorsal  aspect  of  the  kid- 
neys, ureter,  urinary  bladder, 


EXCKETION 


181 


everyday  experience.  The  three  factors  which  increase 
the  activity  of  the  kidneys  in  healthy  people  are  (1) 
exposure  to  cooler  temperature,  (2)  drinking  large  quanti- 
ties of  water,  and  (3)  the  taking  of  proteid  food.  Expo- 
sure to  cold,  or,  more  accurately,  the  cooling  of  the  skin, 
causes  constriction  of  the  cutane- 
ous arterioles  and  a  compensating 
dilation  of  those  of  internal  organs. 
This  means  an  increased  blood-flow 
through  the  kidneys  and  an  in- 
crease in  the  amount  of  urine  se- 
creted. The  absorption  of  water 
or  of  proteid  food  also  increases 
the  activity  of  the  kidneys  by  in- 
creasing the  blood-flow  through 
them. 

These  changes  in  the  quantity 
of  the  urine  secreted  are,  gener- 
ally speaking,  only  changes  in  the 
amount  of  water   rather   than   in  FIG.  77.  Vertical  section  of  the 
the    amount   of   urea,    uric    acid,      kidney'    ^ammatic 

-,.  T       .1          -,.        T        T  ...     The  tubules  (A)    of  the  ffland 

salts,  and  other  dissolved  constit-  open>  on  the  papillje  ^  £)t 
uents.  Certain  constituents  of  the 
urine,  however,  are  not  very  solu- 
ble, so  that  it  is  not  well  to  have  water,  the  only  solvent 
of  these  substances  in  the  urine,  unduly  diminished.  A 
very  scanty  secretion  of  urine  during  the  day  is,  in  general, 
a  distinct  indication,  especially  in  warm  weather,  that  in- 
sufficient water  is  being  taken.  Many  persons  drink  too 
little  water  rather  than  too  much. 

5.  The  Structure  of  the  Skin.  —  The  skin  is  an  organ 
which  performs  several  functions,  the  most  important 
being  (1)  that  of  protecting  the  underlying  structures 
from  drying  and  mechanical  injury ;  (2)  that  of  assisting  in 
maintaining  the  constant  internal  temperature  of  the  body  j 


into  the  pelvis   (<?)  of  the 
ureter  (D) 


182 


THE  HUMAN  MECHANISM 


and  (3)  that  of  receiving  the  external  stimuli  of  pressure, 
heat,  and  cold.  Incidentally,  as  we  have  seen,  the  skin  is 
an  organ  of  excretion.  We  may  there- 
fore describe  its  structure  and  ex- 
cretory function  in  this  connection, 
reserving  the  study  of  its  other  func- 
tions for  Chapters  XII  and  XIV. 

The  skin  consists  of  an  outer  layer, 
the  epidermis,  and  an  inner  layer,  the 
dermis,  cutis,  or  corium.  The  dermis 
consists  of  connective  tissue  richly  sup- 
plied with  blood  vessels,  lymphatics, 
and  nerve  fibers,  together  with  sense 
organs  of  touch.  The  fibers  of  the  der- 
mal connective  tissue  are  most  dense 
near  the  epidermis ;  in  the  deeper  por- 
tions the  network  is  loose  and  the 
lymph  spaces  larger,  the  connective 
tissue  of  the  dermis  passing  insensibly 
into  that  of  the  subcutaneous  connect- 
ive tissue. 

The  cells  of  the  more  open  portions 
of  the  dermal  network,  and  especially 
those  of  the  subcutaneous  tissue,  store 
up  more  or  less  fat  within  their  cyto- 
plasm. The  subcutaneous  tissue,  in- 

deed'    is    °n6    °f    the    m°St    ™P<^nt 

of  epidermis;  c,  duct  organs  in  the  body  for  the  storage  of 


FIG.  78.  Cross  section 
of  skin 


of  sweat  gland ;  D,the    £  ± 
dermis ;  E,  subcutane- 


Connective  tissue  in  which  large 


ous  connective  tissue  amounts  of  fat  are  stored  is  known  as 

(p.  7).    The  blood  ves-    adipose  tjsme   /gee  p.  230). 
sels    are    injected    to  ' 

show  black.  Compare       The  outer  surface  of  the  dermis  is 

not  flat,  but  contains  mound-like  pro- 

jectipns  known  aspapillce,  which  project  into  the  overlying 

epidermis.    Some  of  these  papillae  contain  nerve  endings 


EXCEETION  183 

of  the  sense  of  touch,  while  others  contain  capillaries,  which 
are  found  also  in  other  portions  of  the  dermis.  The  dermis 
is  the  vascular  organ  of>he  skin,  blood  vessels  being  entirely 
absent  from  the  epidermis  (see  Figs.  81,  82). 

The  epidermis  consists  of  many  layers  of  cells,  the  num- 
ber of  layers  being  very  great,  a  hundred  or  more  on  the 
palms  of  the  hands  and  the  soles  of  the  feet ;  in  other 
places  less  exposed  to  pressure  they  may  not  exceed  twenty. 
The  cells  which  rest  immediately  upon  the  dermis  are 
cylindrical  or  columnar  in  shape  ;  those  immediately  above 
them  are  more  or  less  spherical  or  polygonal,  with  minute 
spaces  between  them  for  the  flow  of  lymph.  Both  the 
cylindrical  and  the  spherical  cells  are  alive,  and  it  has 
been  shown  that  they  grow  and  divide.  As  we  pass 
farther  from  the  dermis  with  its  blood  supply  and  nearer 
the  surface  with  its  exposure  to  drying,  the  cells  show 
signs  of  degeneration  and  are  gradually  transformed  into 
dead,  flattened,  horny  scales,  which,  packed  together  and 
known  as  the  horny  layer,  form  the  outer  portion  of  the 
epidermis.  These  scales  are  being  constantly  rubbed  off 
and  their  loss  made  good  by  the  growth  and  multiplication 
of  the  spherical  cells  beneath.  Such  a  covering  or  lining 
is  therefore  well  fitted  for  surfaces  which  are  exposed  to 
friction  or  drying.  The  mouth,  the  part  of  the  pharynx 
used  in  swallowing,  the  oesophagus,  and  the  rectum  are 
lined  with  the  same  tissue.  Afferent  nerves  are  found  in 
the  lower  layers  of  the  epidermis. 

The  hairs,  the  sweat  glands,  and  the  nails  are  modified 
portions  of  the  epidermis.  Of  these  the  hairs  and  the  sweat 
glands  are  of  sufficient  importance  for  our  purposes  to 
merit  some  description. 

6.  Structure  of  a  Hair  and  a  Hair  Follicle. — A  hair  grows 
from  the  bottom  of  a  pit,  the  hair  follicle,  which  extends 
downward  into  the  dermis  or  even  into  the  subcutaneous 
tissue.  Microscopic  examination  shows  that  this  follicle  is 


184 


THE  HUMAN  MECHANISM 


--M 


H 


lined  with  a  continuation  of  the  epidermis,  just  as  a  gland 
of  the  stomach  or  intestine  is  lined  by  an  ingrowth  of  the 
cells  of  its  surface.  At  the  bottom  of  the  follicle  is  a 
papilla,  and  the  hair  which  grows  out 
from  this  papilla  to  the  free  surface 
bears  to  the  cylindrical  and  spherical 
cells  of  the  papilla  the  same  relation 
that  the  horny  layer  of  the  epidermis 
bears  to  the  similar  underlying  cells. 
We  accordingly  find  that  the  hair  is 
composed  of  horny  scales  closely  pressed 
together  into  the  well-known  threadlike 
structure.  The  scales  of  the  outer  layer, 
which  form  the  covering  for  the  hair, 
are,  however,  flattened  and  overlap  some- 
what like  the  shingles  on  the  roof  of  a 
house. 

Opening  into  the  hair  follicle,  one  or 
more  sebaceous  glands  discharge  an  oily 
secretion  which  lubricates  the  hair  and 
the  horny  layer  of  the  epidermis,  and  so 

A,  root  of  hair  ;  B,  bulb  j.     j      •  i      i_          • 

of  the  hair;  C',  inter-    prevents  drying  and  chapping. 

nal  root  sheath  ;  D,  7.    Xhe  Sweat  Glands  are  tubular  pro- 

external  root  sheath;      -,  ,.  p    ,  ,  .  T  .       ,-,  l     ji 

E,  external  mem-    longations  of  the  epidermis  through  the 
brane  of  follicle;  F,    dermis    into    the    subcutaneous    tissue. 

teZd\Tth?feoriiick^    Here  the  tube    becomes    much  coiled, 
H,  compound  seba-    forming  the  secreting  recess,  which  is 


FIG.  79.  Hair  and 
hair  follicle 


baceous  gland;    M,    also  receives  nerves.    It  is  a  simple  tubu- 

LTnci^  °f  the  halr    lar  Sland  formed  as  an  ingrowth  from 

the  epidermis  (see  Figs.  81  and  82). 

8.  The  Secretion  of  the  Perspiration,  like  the  secretion  of 

the  gastric  juice,  is  under  the  control  of  the  nervous  system. 

When  the  nerves  going  to  the  sweat  glands  of  a  given 

area  of  skin  are  cut  or  otherwise  injured,  the  secretion  of 


EXCRETION 


185 


perspiration  ceases  over  that  area;  and  the  appearance  of 
cold  beads  of  perspiration  as  the  result  of  fright  shows  how 
events  taking  place  in  the  nervous  system  may  excite  these 
glands  to  activity  apart  from  the  presence  of  their  nor- 
mal stimuli,  —  the  application  of  heat  to  the  skin  and  the 
liberation  of  heat  within  the 
body  by  muscular  and  other  A 
activities.  The  distinction 
should  be  made  between  the 
so-called  "sensible"  and  "in- 
sensible" perspiration,  the  lat- 
ter name  being  given  to  the 
perspiration  the  water  of  which 
evaporates  as  fast  as  secreted; 
the  former  to  that  which  does 
not  evaporate  so  rapidly  and 
hence  remains  for  a  time  on 
the  surface  of  the  skin.  When 

the  water  evaporates,  the  dis-    FlG-  80-  Magnified  section  of  the 
solved  solids  (salts,  urea,  and 
other  compounds)  remain  be-    A 
hind  on  the  skin. 

9.  Value  of  Profuse  Perspi- 
ration in  the  Care  of  the  Skin. 
-While  the  skin  is  not  pri- 
marily an  organ  of  excretion, 
the  perspiration  contains  a 
certain  amount  of  waste  sub- 
stances and  salts  which  are  left  by  the  evaporation  of  the 
water  upon  the  surface,  and,  to  some  extent,  in  the  mouths 
of  the  ducts  of  the  sweat  glands ;  this  is  especially  the  case 
when  evaporation  takes  place  about  as  rapidly  as  the  per- 
spiration is  discharged.  When  the  secretion  of  perspiration 
is  more  abundant,  as  during  muscular  work,  or  at  very  high 
temperatures,  or,  in  general,  where  it  does  not  evaporate 


lower  portion  of  a  hair  and 
hair  follicle 

membrane  of  the  hair  follicle, 
cells  with  nuclei  and  pigmentary 
granules ;  B,  external  lining  of 
the  root  sheath;  C,  internal  lin- 
ing of  the  root  sheath ;  D,  corti- 
cal or  fibrous  portion  of  the  hair 
shaft;  .#,  medullary  portion 
(pith)  of  shaft;  F,  hair  bulb, 
showing  its  development  from 
cells  from  A 


186 


THE  HUMAN  MECHANISM 


as  rapidly  as  discharged,  the  accumulation  of  solids  in 
the  ducts  of  the  glands  is  washed  out.  For  this  reason  a 
vigorous  perspiration  followed  by  a  bath 
is  a  useful  hygienic  measure  in  the  care 
of  the  skin,  although  it  is  not  necessary, 
as  is  sometimes  supposed,  in  order  to 
secure  the  efficient  elimination  of  wastes 
from  the  blood. 

10.  The  Skin  as  an  Organ  of  Absorp- 
tion.—  While  it  is  true  that  water  as 
perspiration  may  readily  find  its  way  out 
through  the  skin,  such  escape  is  effected 
chiefly  by  the  sweat  glands  which  are 
under  the  strict  control  of  the  nervous 
system.  Apart  from  this  the  skin  is  vir- 
tually water-tight ;  and,  oiled  as  it  is  by 
the  secretion  of  the  sebaceous  glands,  it 
serves  both  to  keep  in  the  water,  which 
forms  so  important  a  part  of  the  tissues, 
and  also  to  keep  out  water  which  might 
otherwise  soak  into  the  body,  as,  for 

This  water- 
proof characteristic  also  makes  it  next 
to  impossible  for  us  to  absorb  food  mate- 
rials by  way  of  the  skin.  A  "milk  bath"  may  be  at  times 
useful  in  the  care  of  the  skin,  because  the  fat  or  oil  of  the 
milk  may  supply  any  deficiency  in  the  sebaceous  secretion 
and  so  insure  lubrication  of  the  epidermis ;  but  it  cannot 
be  regarded  as  a  means  of  supplying  food  to  the  body. 


FIG.  81.  Sweat  gland 
slightly  magnified 

Note  the  coiled  form  of  n         i  i     ,1  • 

the  tube  in  the  sub-    example,  during  bathing. 

cutaneous  tissue. 
Compare  Fig.  82 


CHAPTER  XII 

THERMAL  PHENOMENA  OF  THE  BODY 
A.  THE  CONSTANT  TEMPERATURE 

1.  The  Constant  Temperature. —  No  characteristic  of  the 
human  mechanism  is  more  remarkable  than  its  constant  tem- 
perature.   Whether  we  are  awake  or  asleep,  by  night  or 
by  day,  at  work  or  at  rest,  at  home  or  abroad,  in  summer 
or  winter,  in  the  tropics  or  the  polar  regions,  in  subterra- 
nean caves  or  on  lofty  mountain  peaks,  the  temperature  of 
healthy  human  beings  is  always  nearly  the  same.    So  steady 
is  this  temperature  that  an  increase  or  decrease  of  two  or 
three  degrees  gives  just  cause  for  anxiety,  and  a  change 
of  seven  or  eight  degrees  is  looked  upon  with  alarm. 

In  many  modern  laboratories  constant  temperatures  are 
obtained  by  the  use  of  a  thermostat,  the  apparatus  of  which 
is  visible  and  easily  understood ;  but  no  such  special  appa- 
ratus regulates  the  constant  temperature  of  the  human 
body,  and  we  have  rather  to  seek  an  explanation  in  the 
coordinated  activities  of  organs  already  familiar,  such  as 
muscles,  skin,  blood  vessels,  and  especially  the  all-control- 
ling nervous  system. 

2.  Temperature  and  Chemical  Changes.  —  Every  chemi- 
cal reaction  takes  place  more  readily  under  some  external 
physical  conditions  than  under  others,  and  among  these 
conditions  none  is  more  important  than  temperature.    We 
have  already  seen  this  fact  illustrated  in  the  case  of  the 
enzymes.    At  the  freezing  point,  saliva  exerts  no  action 
upon  starch  paste ;  as  the  temperature  rises,  the  activity 
of  the  enzyme  increases  up  to  a  certain  point,  and  then 

187 


188  THE  HUMAN  MECHANISM 

diminishes  more  or  less  rapidly  until  a  point  is  finally 
reached  at  which  its  peculiar  chemical  properties  are 
destroyed.  (See  Experiment  III,  p.  103.) 

3.  Temperature  and  Vital  Activities. — When  we  come  to 
the  activities  of  living  cells,  —  activities  which,  it  will  be 
recalled,  depend  on  chemical  changes,  —  precisely  the  same 
thing  holds  true,  and  in  so  striking  a  manner  as  to  create  a 
widespread  but  erroneous  impression  that  this  dependence 
upon   temperature    is    peculiarly  characteristic   of   living 
things.    The  one-celled  animal,  Amoeba,  moves  about  more 
actively  and  digests  more  food  at  20°  C.  than  at  10°  C.; 
bacteria  grow  more  rapidly  at  the  room  temperature  than 
near  the  freezing  point ;  the  pitch  of  the  note  made  by  a 
cricket  rises  with  the  temperature,   indicating  that  the 
movements  of  the  wing  covers  which  produce  the  sound 
are  being  made  more  rapidly ;    and  in  the  winter  sleep 
of  hibernating  animals  we  have  a  beautiful  example  of 
the  decline  of  vital  activities  with  the  fall  of  external 
temperature. 

Nor  are  the  living  cells  of  the  human  body  exceptions 
to  this  rule.  The  rate  of  the  heart  beat  varies  directly  with 
the  temperature  of  the  blood,  and  the  character  of  the 
breathing  movements  is  influenced  by  the  same  cause ;  a 
cooled  muscle  contracts  more  slowly,  a  cooled  gland  secretes 
less  abundantly.  If  the  temperature  of  the  body  itself  falls, 
every  vital  activity  is  depressed,  and  death  itself  may 
result  from  undue  cooling. 

4.  The  Constant  Temperature  of  the  Body.  —  This  depres- 
sion of  nervous,  muscular,  and  glandular  activity  results, 
however,  only  from  a  fall  of  the  temperature  of  the  body,  not 
of  that  of  the  surrounding  air  or  other  medium.    These  two 
things  are  by  no  means  the  same,  as  may  be  readily  seen 
from  the  fact  that  a  clinical  thermometer  placed  in  the 
mouth  indicates  almost  the  same  temperature  of  the  body  on 
warm  and  on  cold  days  ;  even  while  we  are  shivering  with 


THERMAL  PHENOMENA  189 

cold,  the  thermometer  gives  about  the  same  reading  as  when 
we  are  enjoying  the  warmest  summer  weather.  The  tem- 
perature of  the  body  remains  nearly  constant,  regardless  of 
changes  in  the  temperature  of  the  air  around  it. 

We  have  only  to  appeal  to  experience  to  see  that  this  is 
not  the  way  in  which  lifeless  matter  generally  behaves  ;  a 
stone,  the  earth,  a  piece  of  iron  is  warmer  on  a  warm  day 
and  colder  on  a  cold  day ;  in  general,  lifeless  things  take 
the  temperature  of  the  medium  in  which  they  are  placed, 
and  this  is  one  of  the  fundamental  principles  of  physics. 
Nor  do  most  living  things  act  differently ;  the  temperature 
of  a  plant  or  a  tree,  of  an  earthworm,  a  frog,  a  turtle,  a 
snake,  does  not  differ  greatly  from  that  of  its  surroundings. 
It  is  only  birds  and  mammals  which  show  this  remarkable 
power  of  maintaining  an  approximately  constant  body 
temperature  notwithstanding  wide  limits  of  change  in  that 
of  the  surrounding  air.  Such  animals  are  known  as  warm- 
blooded because  they  are  usually  warmer  than  surrounding 
objects ;  those  animals  which  do  not  thus  maintain  a  con- 
stant temperature,  on  the  other  hand,  are  known  as  cold- 
blooded.1 

It  is  clear  that  the  power  to  maintain  a  constant  body 
temperature  is  of  the  utmost  importance  in  enabling  an 
animal  to  counteract  the  varying  conditions  of  climate. 
Were  it  not  for  this  power,  man  would  be  a  hibernating 
animal ;  with  the  coming  of  winter  all  his  activities  would 
gradually  be  slowed  down,  and  long  before  our  rivers  and 
ponds  had  begun  to  freeze  all  business,  industrial  life, 
and  intellectual  life  would  come  to  a  standstill ;  it  would 
not  be  possible  for  the  human  race  to  people  every  zone 

1  A  cold-blooded  animal  exposed  to  a  temperature  of  99°  F.  is  as 
warm  as  a  warm-blooded  animal.  Such  animals  are  so  called  because 
they  usually  feel  colder  when  handled  than  do  warm-blooded  animals ; 
but  this  is  merely  because  the  temperature  of  the  air  (which  is  also  their 
temperature)  is  usually  lower  than  the  temperature  of  warm-blooded 
animals. 


190  THE  HUMAN  MECHANISM 

of  the  earth,  —  the  shores  of  Alaska  or  Iceland  as  well  as 
the  banks  of  the  Ganges  or  the  Amazon. 

And  yet  here  again  we  are  dealing  with  a  reaction  which 
is  not  limited  to  man,  or  even  to  mammals  and  birds. 
There  are  some  inanimate  objects  of  human  invention 
which  show  similar  power.  The  automatic  valves  which 
are  used  in  some  of  our  house  furnaces  to  regulate  the 
draft  are  so  arranged  that  as  the  external  temperature  goes 
down  the  draft  is  opened  more  widely  and  the  tempera- 
ture of  the  furnace  remains  the  same  or  even  rises.  The 
best  incubators  for  hatching  eggs  are  provided  with  simple 
devices  which  maintain  a  temperature  as  constant  as  does 
the  hen  sitting  on  her  nest ;  and  we  have  already  referred 
to  the  laboratory  thermostat. 

5.  The  Temperature  of  the  Body  not  absolutely  Constant. 
-  The  term  "  constant "  as  applied  to  the  temperature  of 

warm-blooded  animals  is  not,  however,  to  be  taken  too 
literally.  No  animal  has  an  absolutely  constant  tempera- 
ture. In  the  first  place  there  are  slight  variations  from 
time  to  time  under  the  changing  conditions  of  life.  The 
temperature  is  higher  by  from  one  to  four  degrees  during 
muscular  exercise  than  during  rest;  it  varies  during  the 
day,  being  highest  in  the  afternoon  and  lowest  in  the  small 
hours  of  the  morning  ;  it  is  often  raised  half  a  degree  or 
more  by  taking  food,  and  marked  changes  of  surrounding 
temperature  may  cause  a  change  of  one  degree  or  even 
more  in  that  of  the  body.  These  changes  between  97.5° 
and  99.5°  F.  are  of  everyday  occurrence  and  are  entirely 
normal;  so  that  when  we  speak  of  the  temperature  of  the 
body  being  constant  we  mean  that  it  varies  only  within 
narrow  limits,  or  that  it  is  constant  in  comparison  with 
that  observed  in  cold-blooded  animals. 

6.  The  Temperature  of  Different  Organs.  —  Nor  is  this  all ; 
some  parts  of  the  body  have  a  higher  temperature  than 
others.    Thus  the  temperature  of  the  liver  is  often  as  high 


THERMAL  PHENOMENA  191 

as  107°  F. ;  that  of  the  muscles  varies  between  99°  and 
105°  F.  ;  that  of  the  blood  in  the  right  side  of  the  heart  is 
usually  a  degree  or  so  higher  than  that  of  the  blood  in  the 
left  side.  But  it  is  in  the  skin  that  we  meet  with  the  widest 
variations  from  the  general  average.  Every  one  knows  that 
on  a  very  cold  day  the  temperature  of  the  skin  may  be  far 
below  98.6°  F. ;  indeed,  the  experience  of  "frosted"  ears 
or  feet  shows  that  at  times  cutaneous  temperature  may 
descend  to,  or  even  below  the  freezing  point  itself ;  and 
it  is  very  exceptional  indeed  when  the  skin  temperature 
is  above  92°  or  93°  F.,  even  on  very  hot  summer  days.  These 
variations  are  due  to  the  fact  that  the  skin  is  the  organ 
which  is  immediately  exposed  to  the  changing  environ- 
ment, and  hence  peculiarly  subject  to  cooling  influences. 
It  is  therefore  customary  to  distinguish  between  an  outer 
body  zone  of  variable  temperature  and  the  more  constant 
temperature  of  internal  organs. 

7.  Measurement  of  the  Body  Temperature.  —  The  great 
equalizer  of  the  body  temperature  is  the  blood.  That  which 
has  flowed  through  the  skin  comes  away  cooled ;  that 
which  comes  from  an  organ  like  the  liver  or  a  working 
muscle,  in  which  active  oxidations  or  other  chemical  changes 
have  taken  place,  is  heated.  In  the  great  veins  and  in  the 
heart  the  warmer  blood  is  mixed  with  the  cooler ;  so  that 
theoretically  what  we  mean  by  the  body  temperature  would 
be  that  of  the  blood  which  the  left  side  of  the  heart  pumps 
to  the  organs.  It  is,  of  course,  impracticable  to  measure 
this ;  and  we  must  take  instead  the  temperature  of  organs 
which  are  well  supplied  with  this  blood,  themselves  pro- 
ducing little  or  no  heat,  and  at  the  same  time  not  exposed 
to  cooling  influences.  The  most  accurate  measurements 
of  temperature  are  made  in  the  rectum,  and  there  are  times 
when  reliable  readings  can  be  obtained  only  in  this  way. 
It  is  customary,  however,  to  take  the  temperature  in  the 
mouth,  the  bulb  of  the  thermometer  being  placed  under  the 


192  THE  HUMAN  MECHANISM 

tongue  and  the  lips  kept  closed.  For  many  purposes  this 
is  sufficiently  accurate,  although  at  times  it  may  intro- 
duce a  comparatively  large  error.  Subject  to  the  varia- 
tions mentioned  above,  the  rectal  temperature  is  about 
99.6°  F.,  that  of  the  mouth  being  98.6°. 

8.  The  Feeling  of  Cold  or  Warmth  is  not  a  True  Test  of 
the  Body  Temperature.  —  It  is  well  at  this  point  to  warn  the 
student  against  confusing  the  body  temperature  with  sen- 
sations of  cold  or  warmth.    Since  sensations  of  heat  and 
cold  originate  only  in  the  skin,  the  mouth,  and  perhaps 
the  upper  part  of  the  oesophagus,  —  parts  which  belong  to 
the  zone  of   variable  temperature,  —  it  is  clear  that  our 
feelings  give  us  no  information  as  to  the  temperature  of 
the  internal  parts  of  the  body.    This  fact  is  strikingly  illus- 
trated in  the  case  of  a  "  chill,"  when  the  internal  tempera- 
ture is   almost  always  really  above,  and  not  below,  the 
normal ;  and  the  feeling  of  warmth  produced  by  muscular 
activity  or  by  warming  one's  self  at  a  fire  merely  indicates 
a  higher  temperature  of  the  skin,  not  a  higher  temperature 
of  internal  organs. 

Having  now  learned  the  more  obvious  facts  about  the 
constant  temperature  of  the  body,  we  have  next  to  inquire 
by  what  means  this  constant  temperature  is  maintained. 
Such  an  inquiry  is  of  the  utmost  importance  in  the  study 
of  human  physiology  and  hygiene,  since  the  effort  to  keep 
a  constant  temperature  involves  at  times  the  introduction 
of  physiological  conditions  intimately  related  to  the  main- 
tenance of  the  highest  standard  of  health. 

9.  The   Production  and  the   Transfer   of  Heat.  —  The 
temperature  of  any  body  of  matter  can  remain  constant  only 
as  long  as  the  amount  of  heat  it  produces  or  receives  (from 
objects  warmer  than  itself)  exactly  equals  the  amount  it 
gives  out ;  the  temperature  rises  when  the  heat  produced 
plus  that  received  is  greater  than  that  lost;  and  it  falls 
when  the  balance  of  the  account  is  on  the  other  side. 


THERMAL  PHENOMENA  193 

In  applying  this  principle  to  the  human  body  we  must 
first  remember  that  the  body  produces  or  liberates  heat. 
The  chemical  changes,  largely  oxidative  in  character, 
which  are  at  the  basis  of  the  Work  of  its  muscles,  glands, 
nerve  cells,  etc.,  liberate  heat  just  as  truly  as  the  burning 
of  coal  in  the  furnace  of  an  engine  liberates  heat.  Heat 
production  is  therefore  an  indispensable  result  of  cellular 
and  organic  activity,  and  it  is  greatest  in  those  organs, 
like  the  muscles  and  liver,  which  carry  out  the  most  active 
chemical  processes.  The  body  is  warm  for  the  same  reason 
that  a  stove  is  warm ;  that  is,  because  combustion  (oxidation) 
is  going  on  within  it. 

In  the  second  place,  the  heat  liberated  within  the  body, 
like  that  liberated  in  a  stove,  may  be  transferred  to  sur- 
rounding objects.  Every  one  knows  that  when  a  warm 
body  is  brought  near  a  colder  one,  the  former  becomes 
colder  and  the  latter  warmer.  Heat  has  been  transferred 
from  one  to  the  other,  and  this  transfer  continues  until  the 
two  bodies  reach  the  same  temperature.  The  clothing  is 
warmed  by  contact  with  the  body ;  so  is  the  air  in  imme- 
diate contact  with  the  skin ;  and  conversely  the  body  may 
be  warmed  by  contact  with  anything  warmer  than  itself,  a 
hot-water  bottle,  for  example. 

It  is  not,  however,  necessary  that  two  solid  bodies  be  in 
actual  contact  in  order  that  heat  may  pass  from  one  to  the 
other.  A  stove  warms  all  the  objects  in  a  room,  although 
few  of  them  are  touching  it;  and  the  human  body  may 
lose  heat  to,  or  receive  heat  from,  objects  at  a  greater  or 
less  distance.  There  are,  indeed,  three  methods  of  trans- 
ferring heat  from  one  body  to  another,  and  before  we  go 
farther  it  is  important  that  these  methods  of  heat  transfer 
be  thoroughly  understood.  They  are  known  as  conduction, 
convection,  and  radiation.  In  the  case  of  conduction  and 
convection  the  means  of  transferring  heat,  or  the  "  vehicle  " 
of  transfer,  is  some  form  of  matter,  either  a  solid,  or  a 


194  THE  HUMAN  MECHANISM 

liquid,  or  a  gas ;  radiation,  on  the  other  hand,  occurs  inde- 
pendently of  the  presence  of  ordinary  matter  between  the 
two  bodies,  though  on  our  earth  the  air,  at  least,  is  almost 
always  present. 

10.  Conduction. —  Whenever  heat  is  transferred  directly 
from  one  mass  of  matter  to  another  with  which  it  is  in 
contact,  such  transfer  is  known  as  conduction.    A  good 
example  is  the  heating  of  a  poker  in  a  fire  ;  the  heat  of 
burning  coal  is  communicated  directly  to  the  outer  par- 
ticles of  iron,  and  then  from  one  particle  of  the  iron  to 
another.    The  particles  of  iron  do  not  move  up  and  down 
the  length  of  the  poker;    each  one  simply  passes  on  to 
the  next  the  heat  it  has  received,  and  finally  those  of  the 
handle  communicate  their  heat  to  the  hand.    All  transfer 
of  heat  along  solid  objects,  or  from  one  mass  of  matter  to 
another  with  which  it  is  in  immediate  contact,  is  by  means 
of  conduction. 

Solids  and  liquids  are  much  better  conductors  of  heat 
than  gases,  and  air  when  perfectly  still  is  one  of  the 
poorest  conductors  of  heat  with  which  we  have  to  deal. 
It  is  a  familiar  fact  that  the  skin  is  chilled  much  more 
rapidly  by  water  than  by  air  of  the  same  temperature 
(why  ?) ;  and  we  shall  learn  in  hygiene  that  warm  fabrics 
owe  their  warmth  mainly  to  the  amount  of  poor-conducting 
air  stagnant  within  their  meshes. 

11.  Convection.  —  When  a  warm  body  is  surrounded  by 
a  fluid  such  as  water  or  air,  heat  is  similarly  conducted  to 
the  adjacent  layer  of  water  or  air,  which  thus  becomes 
warmer;   but,  unlike  the  case   of  the   solid,  this  heated 
layer  now  moves  off,   carrying  its  heat  with  it  to  other 
parts  of  the  gas  or  liquid,  and  so  communicating  it  to 
other  matter  with  which  it  subsequently  comes  in  contact. 
This  method  of  heat  transfer  is  known  as  convection,  which, 
it  will  be  seen,  depends  at  bottom  upon  conduction,  but 
which  is  at  the  same  time   conduction  modified  by  the 


THEEMAL  PHENOMENA  195 

movement  of  a  heated  gas  or  liquid.  So  long  as  the  air 
around  us  is  at  rest  it  does  not  remove  heat  readily  from 
the  skin,  since  air  is  a  poor  conductor.  Air  in  motion,  on 
the  other  hand  (as  in  fanning),  cools  the  skin  more  rapidly, 
because  as  each  part  of  the  air  is  heated  it  is  moved  away 
and  replaced  by  colder  air.  In  this  case  the  air  cools  the 
skin  by  convection  (Latin  con-,  with;  vehere,  to  carry). 

The  transfer  of  heat  from  the  internal  heat-producing 
organs  to  the  skin  affords  an  excellent  example  of  the 
difference  between  conduction  and  convection;  for  some 
of  this  heat  passes  by  direct  conduction  through  the  sub- 
cutaneous tissue  to  the  overlying  skin,  while  some  of  it  is 
carried  to  the  surface  by  convection  in  the  blood  stream. 
When  the  arterioles  of  the  skin  are  dilated,  convection  is 
an  important  means  of  heat  transfer  to  the  surface ;  when, 
in  the  reverse  case,  the  cutaneous  arterioles  are  constricted 
to  their  utmost,  convection  becomes  relatively  unimpor- 
tant and  direct  conduction  alone  remains  as  the  chief  means 
of  heat  transfer  to  the  skin.  Moreover,  when  the  subcuta- 
neous tissue  contains  large  amounts  of  fat,  it  is  a  poor  con- 
ductor of  heat,  and  for  this  reason  fat  people  when  sitting 
still  on  cold  days  often  feel  colder  than  lean  people  do. 

12.  Radiation.  —  Heat  is  thus  removed  from  the  skin 
by  conduction,  and  at  times  to  an  even  greater  extent  by 
convection.  But  there  is  still  a  third  method  of  heat  loss, 
known  as  radiation,  by  which  heat  can  be  transferred  across 
a  space  in  which  there  is  neither  solid,  liquid,  nor  gas, 
and  in  which  conduction  and  convection  are  consequently 
impossible.  The  most  familiar  and  striking  example  of 
radiation  is  the  transfer  of  heat  from  the  sun  to  the  earth, 
since  there  is  no  atmosphere  in  the  greater  part  of  the 
more  than  ninety  millions  of  miles  of  space  which  separate 
us  from  that  intensely  heated  body. 

Any  detailed  consideration  of  radiation  belongs  to  the 
domain  of  physics  rather  than  physiology,  and  would  be 


196  THE  HUMAN  MECHANISM 

out  of  place  here.  It  is  enough  for  our  present  purposes 
to  understand  that,  whether  a  solid  body  be  in  an  atmos- 
phere of  air,  or  in  a  transparent  liquid,  or  even  in  a  vacuum, 
it  transfers  or  loses  heat  by  direct  radiation  to  colder 
objects  about  it.  From  an  open  fire  heat  may  be  trans- 
ferred by  conduction  to  andirons  or  walls  in  direct  contact 
with  it;  or  by  convection  through  heated  air  currents  to 
the  chimney  top ;  or,  finally,  by  radiation  to  persons  stand- 
ing in  front  of  it.  In  the  latter  case  the  heating  is  chiefly 
by  radiation,  since  there  is  no  contact  with  the  fire,  and 
such  air  currents  as  exist  are  mostly  composed  of  cool 
air  sucked  towards  and  into  the  chimney  by  its  draft.  It 
is  for  these  reasons  that  open  fires  are  said  to  "roast  people 
in  front  and  freeze  them  behind."  Conversely,  the  human 
body,  if  warmer  than  its  surroundings,  may  lose  heat  by 
conduction,  convection,  and  radiation  to  cooler  objects  in 
the  vicinity. 

The  practical  importance  of  these  facts  is  seldom  real- 
ized. It  often  happens  that  the  air  in  contact  with  the 
skin  is  of  the  proper  room  temperature  ;  and  yet,  if  one 
is  sitting  too  near  a  cold  wall  or  window,  enough  heat 
may  be  lost  by  radiation  from  the  skin  to  the  cold  wall, 
through  the  warm  air,  to  chill  the  skin  materially,  causing 
a  loss  of  heat  and  a  "cold." 

13.  Laws  of  Conduction  and  Radiation.  —  For  our  pur- 
poses the  two  most  important  factors  which  determine  the 
loss  of  heat  by  conduction  and  radiation  are  (1)  the  differ- 
ence in  the  temperature  of  the  two  objects  ;  and  (2)  the 
distance  between  them.  In  general,  the  greater  the  dif- 
ference of  temperature  the  more  heat  will  be  lost  from 
the  warmer  to  the  colder  object ;  thus  the  skin  loses  heat 
rapidly  by  these  means  when  surrounding  objects  are  at 
0°  F.,  but  only  slowly  when  they  are  at  90°.  It  is  also 
clear  that  as  soon  as  the  temperature  of  surrounding  ob- 
jects and  of  the  atmosphere  is  as  high  as  that  of  the 


THEEMAL  PHENOMENA  197 

body  (98.6°  F.)  no  further  heat  can  be  lost  by  conduction 
and  radiation ;  and  that  above  98.6°  F.  heat  is  conducted 
and  radiated  to  the  body,  not  from  it. 

Furthermore,  the  greater  the  distance  of  the  colder  ob- 
ject from  the  body  the  less  heat  will  the  body  lose*  to  it. 
Here  heat  loss  takes  place  inversely  as  the  square  of  the 
distance ;  i.e.  when  we  are  twice  as  far  away  from  a  cold 
(closed)  window  we  lose  only  one  fourth  as  much  heat 
through  it  by  radiation ;  if  we  are  three  times  as  far  away, 
we  lose  only  one  ninth  as  much,  and  so  on.  Consequently 
we  rapidly  diminish  radiation  from  our  bodies  by  sitting 
farther  away  from  the  walls  of  a  room  ;  and  it  is  important 
to  have  our  living  rooms  large  enough  to  make  it  unneces- 
sary to  sit  near  the  windows  or  near  a  cold  outer  wall  in 
very  cold  weather. 

14.  Heat  Output  by  the  Evaporation  of  Perspiration.  - 
In  one  other  way  heat  may  be  lost  from  the  body.  The 
change  from  the  liquid  to  the  gaseous  state  (evaporation) 
always  involves  absorption  of  heat  from  surrounding  ob- 
jects. This  is  readily  illustrated  by  the  simple  experi- 
ment of  blowing  a  steady  current  of  dry  air  over  the  dry 
hand,  and  comparing  the  cooling  thus  produced  by  con- 
vection with  that  produced  by  blowing  the  same  current 
against  the  moistened  finger.1  The  cooling  in  the  latter 
case  will  be  much  greater  than  in  the  former,  and  is  caused 
by  the  evaporation  of  the  water.  Liquids  like  ether,  which 
evaporate  more  rapidly  than  water,  will  produce  even 
greater  feeling  of  cold  on  the  skin. 

In  the  secretion  of  perspiration  we  have  this  principle 
applied  to  the  uses  of  the  body.  The  student  should 
understand  clearly  at  the  outset,  however,  that  it  is  the 
evaporation  of  the  perspiration,  not  the  secretion  of  it, 
which  abstracts  heat  from  the  body.  Perspiration  may  be 

1  In  this  experiment  the  temperature  of  the  room  and  of  the  current 
of  air  must  be  so  low  that  they  will  not  induce  active  perspiration. 


198  THE  HUMAN  MECHANISM 

secreted  in  large  quantities,  but  if  it  does  not  evaporate,  — 
as  happens  on  a  very  moist,  humid,  muggy  day,  when  the 
atmosphere  already  contains  about  as  much  aqueous  vapor 
as  it  can  hold,  —  it  takes  little  or  no  heat  from  the  skin. 
Nor  is  the  efficiency  of  the  perspiration  as  a  cooling  agent 
measured  by  the  amount  of  visible  or  "  sensible  "  perspira- 
tion, for  this  is  only  the  perspiration  which  has  not  evap- 
orated ;  the  true  measure  of  the  cooling  effect  would  be 
the  perspiration  which  has  evaporated  and  of  which  we 
are  not  conscious. 

It  is  important  to  note  that  the  evaporation  of  perspira- 
tion (or  of  water  from  the  lungs  and  air  passages)  is  the 
only  means  of  cooling  the  body  when  objects  around  it 
are  warmer  than  the  body  itself.  In  this  case  conduction, 
convection,  and  radiation  only  add  heat  to  the  body ;  but 
even  their  combined  action  may  often  be  overcome  by  an 
abundant  evaporation  of  perspiration.  Men  have  remained 
for  some  time  in  rooms  whose  temperature  was  as  high  as 
260°  F.,  or  48°  above  the  boiling  point  of  water,  without 
any  marked  rise  of  the  body  temperature  and  without 
severe  discomfort,  the  temperature  of  the  body  being  kept 
down  solely  by  the  evaporation  of  perspiration  from  the 
skin.  In  order  to  make  this  means  of  cooling  possible,  it 
is  absolutely  essential  that  the  air  be  dry  and  capable  of 
taking  up  moisture.  No  one  can  survive  long  at  such  tem- 
peratures in  moist  air. 

15.  Summary.  —  In  conclusion  we  may  recapitulate  the 
ways  in  which  the  body  may  gain  or  lose  heat. 

It  may  GAIN  heat  by  conduction,  convection,  or  radiation 
from  warmer  surroundings.  The  amount  of  heat  it  receives 
from  without  is,  however,  very  small  compared  with  that 
derived  from  the  oxidations  taking  place  within  itself. 
These  oxidations  constitute  the  chief  sources  of  its  heat. 

It  may  LOSE  heat  (1)  by  conduction,  convection,  or  radia- 
tion to  things  external  to  itself. »  Here  are  included  not 


THERMAL  PHENOMENA  199 

only  the  heat  lost  from  the  skin  but  also  that  lost  in  heat- 
ing the  expired  air  and  the  excretions ;  (2)  by  evaporation 
of  water  from  the  skin,  the  lungs,  and  the  air  passages. 

It  is  quite  impossible  to  give  definite  figures  showing 
just  what  percentage  of  the  total  heat-loss  occurs  by  each 
of  these  channels,  because  this  differs  at  different  times ; 
on  a  very  cold  day,  for  example,  none  is  lost  in  perspiration 
because  no  perspiration  is  secreted,  but  much  is  lost  by 
radiation  and  convection ;  on  a  hot  day  the  conditions  are 
reversed.  On  a  warm  day  when  the  wind  is  blowing  con- 
vection plays  a  more  important  r61e  than  it  does  when  the 
air  is  quiet.  During  vigorous  exercise  on  a  cool  day  we 
lose  heat  by  evaporation  of  perspiration ;  when  we  are  at 
rest  at  the  same  temperature  little  or  no  heat  is  lost  by 
this  means.  A  little  reflection  will  show  that  most  of  the 
output  of  heat  from  the  body  is  from  the  skin.  The  heat 
which  the  body  loses  in  the  urine  and  feces  is  almost  negli- 
gible, and  that  which  is  lost  by  heating  the  expired  air 
and  evaporating  the  water  which  this  air  carries  off  as 
aqueous  vapor  probably  never  exceeds  ten  or  twelve  per 
cent  of  the  whole.  From  eighty-five  to  ninety  per  cent  of 
the  total  output  of  heat  is  by  way  of  the  skin  ;  so  that  it 
need  not  surprise  us  to  find  that  it  is  the  skin  which  plays 
by  far  the  most  important  rdle  in  checking  or  facilitating, 
as  the  case  may  be,  the  output  of  heat. 

B.  THE  REGULATION  OF  THE  BODY  TEMPERATURE 

In  the  preceding  section  we  have  learned  the  essential 
facts  concerning  the  production  and  output  of  heat  by  the 
human  body,  and  have  also  seen  that  a  constant  tempera- 
ture is  maintained  because  heat  production  and  output  are 
kept  equal.  We  have  now  to  study  the  means  employed 
to  adjust  heat  production  to  heat  loss,  and  heat  loss  to  heat 
production.  For  this  purpose  we  may  begin  by  considering 


200  THE  HUMAN  MECHANISM 

what  happens  during  the  two  conditions  of  daily  life  which 
most  conspicuously  threaten  the  heat  balance,  i.e.  during 
changes  in  external  climatic  conditions  and  during  internal 
conditions  produced  by  muscular  activity.  The  former 
disturbs  the  balance  by  changing  the  rate  of  conduction, 
convection,  radiation,  and  evaporation  from  the  surface; 
the  latter,  by  producing  more  heat  within  the  body. 

16.  Reactions  of  the  Body  at  Rest  to  Changes  of  External 
Temperature.  —  The  more  important  climatic  conditions 
which  affect  the  transfer  of  heat  are  temperature,  atmos- 
pheric moisture  (or  humidity),  and  wind.  We  may  study 
first  the  influence  of  changes  of  temperature,  assuming  for 
the  present  that  the  air  remains  of  moderate  humidity  and 
that  there  is  little  or  no  wind. 

On  a  warm  day,  when  the  thermometer  stands  at  90°  F. 
or  more,  and  one  is  lightly  clad  and  sitting  still,  perspira- 
tion is  secreted  abundantly  and  the  arterioles  of  the  skin 
are  widely  dilated.  Let  us  now  suppose  that  the  day  grad- 
ually grows  cooler.  The  perspiration  becomes  less  abun- 
dant, and,  as  the  falling  temperature  nears  70°  F.,  ceases 
to  be  of  consequence  ;  but  the  blood-flow  through  the  skin, 
though  lessened,  is  still  considerable.  Below  70°  the  per- 
spiration ceases,  so  that  heat  is  now  lost  almost  entirely 
by  conduction,  convection,  and  radiation. 

As  the  temperature  falls  below  70°  F.,  the  veins  are  no 
longer  conspicuous  on  the  hand  and  arm,  which  shows  that 
the  loss  of  heat  is  being  checked  by  limiting  the  blood 
supply  to  the  skin.  At  the  same  time  the  arterioles  of 
internal  organs  are  dilating  (see  p.  150),  so  that  the  liver, 
the  kidneys,  the  mucous  membranes  of  the  alimentary 
canal,  and  those  of  the  air  passages  contain  an  increased 
amount  of  blood. 

By  the  time  the  temperature  has  fallen  to  60°  F.,  or 
thereabouts,  the  cutaneous  arterioles  have  constricted  to 
their  utmost,  the  blood-flow  through  the  skin  has  nearly 


THEEMAL  PHENOMENA  201 

ceased,  and  the  organism  has  no  means  at  command  by 
which  to  restrict  the  further  output  of  heat.  If  in  this 
emergency  heat  production  were  to  remain  constant  while 
external  temperature  continued  to  fall,  the  temperature  of 
the  body  would  be  lowered,  for  conduction  and  radiation 
not  only  continue  but  increase.  That  it  is  not  usually 
lowered  is  due  solely  to  the  fact  that  more  heat  is  then 
produced  within  the  body;  the  oxidations  (and  hence  heat 
production)  which  have  remained  fairly  constant  in  amount 
between  90°  F.  and  65°  F.  external  temperature  now  in- 
crease to  compensate  the  inevitable  loss,  and  continue  to 
increase  as  the  atmospheric  temperature  continues  to  fall. 

17.  The  "  Dangerous  Region  "  of  Atmospheric  Tempera- 
ture.—  It  is  important  to  understand  that  the  methods  of 
regulating  the  temperature  of  the  body  differ  with  differ- 
ent external  temperatures.  Below  60°  the  only  available 
method  is  to  vary  heat  production ;  above  98.6°  the  sole 
method  is  to  evaporate  perspiration  ;  between  70°  and  98.6° 
the  evaporation  of  perspiration  is  combined  with  variations 
in  the  blood-flow  through  the  skin ;  and  finally  we  have 
left  the  region  between  60°  and  70°,  when  the  vascular 
changes  in  the  skin  play  the  most  important  r61e. 

In  this  region  between  60°  and  70°  three  or  four  degrees 
make  all  the  difference  between  the  ideal  room  tempera- 
ture and  one  which  is  decidedly  dangerous.  At  66°  or  68° 
the  blood  is  properly  distributed  between  the  skin  and  the 
internal  organs,  and  there  is  no  excess  in  either.  At  60° 
or  61°,  on  the  other  hand,  the  blood  is  forced  back  upon 
the  internal  organs,  thus  threatening  serious  congestions 
and  other  unhealthful  conditions  which  we  shall  consider 
at  length  in  our  study  of  hygiene  (see  Part  II,  Chap- 
ter XXI).  It  is  because  the  temperature  of  a  room  may 
fall  from  66°  to  60°  so  gradually  that  we  do  not  notice  it 
until  the  internal  damage  is  done,whereas  it  could  not  fall 
to  50°  or  40°  without  our  noticing  it  and  correcting  the 


202  THE  HUMAN  MECHANISM 

trouble,  that  more  colds  are  taken  in  the  former  case  than 
in  the  latter.  In  other  words,  as  the  temperature  goes 
below  65°  the  body  seems  at  first  to  rely  wholly  on  the 
vascular  mechanism  of  temperature  regulation,  and  does  not 
begin  to  produce  more  heat  until  this  resource  has  been 
utilized,  not  only  to  its  utmost,  but  even  to  an  extent  incon- 
sistent with  health.  The  "  dangerous  region  of  room  tem- 
perature," then,  may  be  placed  between  60°  F.  and  65°  F.1 

18.  The  Influence  of  Atmospheric  Moisture  or  "  Humid- 
ity." —  The  amount  of  water  vapor  which  the  atmosphere 
contains  influences  the  output  of  heat  in  two  ways.  In  the 
first  place  moist  air  conducts  heat  from  the  skin  more 
readily  than  dry  air.  We  do  not  notice  this  action  of  moist 
air  in  very  warm  weather  because  the  difference  of  tempera- 
ture between  the  body  and  the  air  is  then  so  slight  that 
neither  moist  nor  dry  air  conducts  any  large  amount  of  heat 
from  the  surface.  But  when  this  difference  of  temperature 
is  greater,  the  effect  of  humidity  in  facilitating  conduc- 
tion becomes  quite  evident.  Moist  air  at  65°  F.  is  distinctly 
chilly  when  one  is  sitting  still,  while  dry  air  at  this  tem- 
perature is  comfortable. 

As  soon,  however,  as  the  evaporation  of  perspiration 
becomes  necessary  to  maintain  the  constant  temperature, 
the  presence  of  moisture,  so  far  from  aiding  the  output  of 
heat,  actually  interferes  with  it ;  for  evaporation  will  not 
take  place  readily  into  moist  air.  This  is  the  reason  why 
a  high  degree  of  humidity  makes  us  feel  warmer  on  a 
warm  day,  whereas,  as  stated  above,  it  also  makes  us  feel 
colder  on  a  cold  day.  In  the  latter  case  it  acts  by  conduct- 
ing heat  more  rapidly  from  the  skin;  in  the  former  by 
interfering  with  evaporation,  which  is  then  the  chief  means 
of  heat  output. 

1  The  student  is  reminded  that  in  the  above  discussion  the  figures  given 
apply  only  when  one  is  lightly  clad  and  sitting  still,  when  the  humidity 
is  moderate,  and  when  the  air  is  comparatively  quiet. 


THEEMAL  PHENOMENA  203 

19.  The  Effect  of  Stagnant  vs.  Moving  Air ;   the  Aerial 

Blanket On  a  perfectly  still  day  the  layer  of  air  about 

the  body  becomes  warmed  by  the  skin,  and,  so  long  as  it  is 
not  removed,  forms  an  air-blanket  which  goes  far  to  keep  the 
skin  warm ;  for  air  is  a  poor  conductor  of  heat.    As  soon, 
however,  as  a  breeze  springs  up  convection  comes  into  play 
and  the  skin  is  cooled  more  rapidly.    In  stagnant  air,  more- 
over, the  evaporation  of  the  perspiration  tends  to  saturate 
this  air-blanket  with  water  vapor,  so  that  further  evaporation 
is  rendered  difficult.    Accordingly,  when  perspiration  is  not 
being  secreted,  moving  air  cools  the  body  by  increasing 
convection  ;  and  when  the  skin  is  moist  it  cools  the  body 
both  by  increasing  convection  and  by  facilitating  the  evapo- 
ration of  perspiration. 

20.  The  Influence  of  Muscular  Activity.  —  External  tem- 
perature, moisture,  and  wind  tend  to  disturb  the  balance 
of  the  heat  account  chiefly  by  changing  the  rate  at  which 
heat  is  lost  from  the  body.    But  the  balance  may  also  be 
disturbed  by  changing  the  amount  of  heat  liberated  within 
the  body.    This  occurs  most  conspicuously  during  muscu- 
lar activity,  for  the  chemical  changes  (or  oxidations)  which 
then  take  place  necessarily  involve  the  liberation  of  large 
amounts  of  heat  within  the  muscles ;  and  this  heat  must 
evidently  be  got  rid  of,  if  a  constant  temperature  is  to  be 
maintained. 

The  reactions  which  then  occur  are  familiar  to  every  one. 
The  arterioles  of  the  skin  are  dilated  (while  those  of  internal 
organs  are  constricted)  and  perspiration  is  secreted.  These 
are  the  same  reactions  which  are  noticed  when  the  body  is 
exposed  to  external  warmth,  and  their  purpose  is  the  same 
in  both  cases,  —  to  facilitate  the  escape  of  heat.  But  in 
the  one  case  they  are  made  necessary  by  the  fact  that 
climatic  conditions  interfere  with  the  output  of  heat,  in  the 
other  by  the  fact  that  more  heat  is  being  liberated  and 
hence  more  must  be  got  rid  of. 


204  THE  HUMAN  MECHANISM 

Seldom  indeed  is  so  severe  a  strain  imposed  upon  the 
mechanism  of  heat  dissipation  as  during  vigorous  muscular 
exertion,  and  especially  when  the  external  conditions  are 
not  favorable  for  radiation,  convection,  and  the  evaporation 
of  perspiration.  Caution  is  then  urgently  indicated  lest 
we  make  the  strain  too  great.  It  is  a  practical  point  to 
remember  in  this  connection  that  some  forms  of  muscular 
exertion  introduce  conditions  for  getting  rid  of  the  surplus 
heat  much  more  readily  than  others ;  this  is  especially  true 
of  those  which  involve  movement  of  the  body  as  a  whole. 
Bicycle  or  horseback  riding,  by  creating  a  breeze,  renders 
the  cooling  of  the  body  a  much  easier  matter  than  does 
sawing  wood,  or  swinging  Indian  clubs,  or  gymnastic  work 
in  general ;  again,  a  particular  form  of  exercise  on  a  dry 
day,  when  the  perspiration  can  evaporate  readily,  may  be 
safe,  while  it  would  be  decidedly  inadvisable  on  a  muggy 
day,  even  though  the  temperature  were  somewhat  lower. 
Indeed,  by  this  time  the  student  must  have  learned  that 
the  thermometer  alone  is  no  safe  indicator  of  the  difficulty 
of  heat  elimination  in  warm  weather. 

21.  Relations  of  Climatic  Conditions  to  Mental  Work 
and  Sleep.  —  During  mental  work  the  brain  requires  an 
increased  supply  of  blood,  and  this  is  obtained  partly  by 
diminishing  the  supply  to  the  skin  (constriction  of  cutane- 
ous arteries) ;  during  sleep,  on  the  other  hand,  the  supply 
to  the  brain  is  diminished,  and  this  is  ordinarily  effected 
by  dilating  the  arteries  of  the  skin  (see  p.  153).  Mental 
work  is  difficult  on  very  warm  days,  partly  because  it  is  dif- 
ficult to  bring  about  cutaneous  constriction ;  and  it  is 
especially  difficult  on  warm,  muggy  days,  since  the  mainte- 
nance of  the  constant  temperature  then  requires  an  exces- 
sive cutaneous  dilation,  and  the  brain  is  quite  unable  to 
command  its  needed  blood  supply. 

During  sleep,  on  the  other  hand,  the  arterioles  of  the 
skin  should  dilate,  and  this  they  cannot  readily  do  when 


THERMAL  PHENOMENA  205 

the  skin  is  exposed  to  cold.     To  "  sleep  warm  "  is  good 
advice,  and  is  based  on  sound  physiological  principles. 

22.  Digestion  and  the  Regulation  of  the  Constant  Tempera- 
ture.—  During  digestion,  and  especially  during  its  earlier 
stages,  when  secretion  is  at  its  maximum,  a  large  supply 
of  blood  is  needed  in  the  stomach,  the  pancreas,  and  the 
intestine.     This  cannot  readily  be  secured  when  the  blood 
is  being  sent  in  large  quantities  to  the  skin  in  order  to  cool 
the  body.    We  have  seen  all  along  that  the  two  great  vascu- 
lar areas  of  the  skin  and  digestive  organs  are  more  or  less 
antagonistic  or  compensating  in  their  vasomotor  reactions. 
When  the  blood  is  present  in  large  quantities  in  the  skin 
it  is  present  in  smaller  quantities  in  the  stomach,  the  intes- 
tine, the  pancreas,  the  liver;  and,  vice  versa,  these  organs 
can  best  obtain  an  adequate  blood  supply  when  the  demands 
of  the  skin  are  not  excessive.     Consequently  digestion  is 
more  difficult  in  warm  than  in  cold  we.ather,  and  we  should 
then  eat  less  at  a  time,  even  if  we  have  to  eat  somewhat 
more  frequently. 

During  the  digestion  of  a  meal  the  chemical  activities  of 
secretion,  the  peristaltic  muscular  movements,  etc.,  some- 
what increase  heat  production  in  the  body;  and  this  in- 
crease, though  not  great,  is  at  times  great  enough  to  make 
us  feel  distinctly  warmer.  When  one  is  slightly  chilly, 
for  example,  he  often  feels  warmer  after  eating  some- 
thing, even  though  the  meal  be  cold;  and  on  a  very 
warm,  muggy  day,  when  the  blood-flow  through  the  skin  is 
already  excessive  and  its  temperature  unduly  high,  the 
digestion  of  a  meal  often  adds  to  the  discomfort,  because 
the  larger  production  of  heat  leads  to  further  dilation  of 
the  skin  vessels. 

23.  Subcutaneous  Adipose  Tissue  a  Hindrance  to  the  Out- 
put of  Heat.  —  In  the  last  chapter  we  saw  that  heat  may 
be  brought  from  the  internal  heat-producing  organs  to  the 
skin  in  two  ways,  —  by  the  circulation  (convection),  or  else 


206  THE  HUMAN  MECHANISM 

by  direct  conduction  through  the  tissues.  How  much  will 
take  one  course  and  how  much  the  other  depends  partly 
upon  the  conducting  power  of  the  tissues  in  question.  Fat 
is  a  poor  conductor  of  heat ;  and  when  it  is  stored,  as  is 
usual  in  fat  people,  in  large  quantities  in  the  connective 
tissue  immediately  under  the  skin,  it  so  interferes  with 
direct  heat  conduction  that  a  larger  proportion  of  the 
transfer  of  heat  to  the  surface  must  be  by  the  circulation ; 
the  cutaneous  arterioles  must  be  more  widely  dilated  and 
the  perspiration,  if  necessary,  must  be  increased.  Hence 
muscular  exertion  and  exposure  to  warm  weather  produce 
more  discomfort  in  fat  than  in  lean  people  because  of  the 
larger  quantity  of  perspiration  and  the  heating  of  the  skin 
by  the  greater  cutaneous  blood-flow ;  such  people  readily 
become  "  overheated."  On  the  other  hand,  contrary  to  pop- 
ular belief,  when  fat  people  are  exposed  to  cold  weather 
they  are  often  more  chilly  when  sitting  still  than  lean 
people,  probably  because,  since  the  blood-flow  through  the 
skin  is  lessened,  their  skin  has  not  the  same  source  of 
warmth  in  the  direct  conduction  of  heat  from  subjacent 
tissues.  Fat  people  thus  retain  the  heat  of  the  body  bet- 
ter, and  should  maintain  more  easily  the  constant  inter- 
nal temperature;  but  this  very  retention  of  heat  within 
the  internal  organs  keeps  heat  away  from  the  skin,  which 
consequently  becomes  cold  and  produces  the  feeling  of 
chilliness. 

24.  The  Mechanism  of  Temperature  Regulation.  —  The 
preceding  pages  have  shown  us  that  temperature  regula- 
tion depends  chiefly  on  three  physiological  mechanisms  : 
(1)  the  vasomotor  system,  which  controls  the  distribution 
of  blood  between  the  skin  and  the  internal  organs ;  (2)  the 
sweat  glands  ;  (3)  the  mechanism  of  heat  production.  The 
first  of  these  has  already  been  described  in  the  study  of 
the  circulation.  The  heating  of  the  skin  stimulates  afferent 
nerves  which  reflexly  dilate  the  arteries  of  the  skin  and 


THEEMAL  PHENOMENA 


207 


also  simultaneously  constrict  those  of  internal  organs.  This 
reflex,  then,  is  dependent  on  the  temperature  of  the  skin ; 
anything  which  heats  the  skin  causes  a  reflex  dilation  of 
its  arterioles  and  lessens  the  supply  of  blood  to  internal 
organs. 

The  secretion  of  perspiration  is  also  under  the  control 
of  the  nervous  system.    The  sweat  glands,  like  the  salivary 


Nerve  Endings  affected 
by  Warmth 


FIG.  82.  Diagram  of  the  cutaneous  reflexes  of  temperature  regulation 

Showing  the  epidermis,  blood  vessels  of  the  dermis,  a  sweat  gland,  and  the 
nervous  mechanism  governing  blood-vessels  and  sweat  glands 

glands,  receive  nerves,  and  secrete  only  in  response  to  their 
stimulation.  When  the  nerves  going  to  the  sweat  glands 
of  any  region  are  injured,  exposure  of  these  glands  to 
external  warmth  produces  no  perspiration ;  stimulation  of 
their  nerves,  however,  produces  a  copious  secretion. 

25.  The  Skeletal  Muscles  the  Main  Organs  in  the  Regu- 
lation of  Heat  Production.  —  The  third  mechanism  of  heat 
regulation  is  that  whereby  the  amount  of  heat  produced  is 
increased  as  it  is  needed,  and  experiment  has  shown  that 


208  THE  HUMAN  MECHANISM 

the  main  organs  here  concerned  are  the  skeletal  muscles. 
When  these  are  in  any  way  prevented  from  contracting, 
the  increase  of  heat  production  on  exposure  to  cold  does 
not  occur. 

Certain  drugs,  for  example,  interrupt  the  connection 
between  muscles  and  their  motor  nerves,  and  when  an 
animal  under  their  influence  is  exposed  to  a  falling  tem- 
perature the  increase  of  heat  production  below  60°  F.  is 
not  observed  ;  the  temperature  of  the  body  rapidly  falls. 
Similarly,  experiment  has  shown  that  if  a  human  being, 
exposed  to  a  falling  temperature,  is  careful  to  lie  in  as 
relaxed  a  condition  as  possible,  thus  suppressing  as  far  as 
possible  the  activity  of  the  skeletal  muscles,  the  usual  heat 
production  fails  to  appear. 

From  these  facts  we  conclude  that  below  60°  F.  cold 
stimulates  cutaneous  afferent  nerves,  which  reflexly  excite 
the  skeletal  muscles  of  the  body,  and  that  the  heat  pro- 
duced by  their  contraction  tends  to  compensate  the  in- 
creased loss  of  heat  from  the  skin. 

26.  Shivering.  —  The  contraction  of  the  muscles  thus 
produced  does  not  necessarily  result  in  movement.  Almost 
every  joint  at  which  motion  ordinarily  takes  place  is  acted 
upon  by  antagonistic  muscles;  for  example,  the  elbow, 
finger,  knee,  and  ankle  joints  are  moved  by  extensor  and 
flexor  muscles,  but  only  when  one  group  is  contracting  to 
a  greater  extent  than  the  other.  If  the  two  groups  con- 
tract to  the  same  extent,  the  action  of  the  one  neutralizes 
that  of  the  other,  so  far  as  movement  is  concerned  ;  but 
both  contract,1  and  in  contracting  produce  heat.  This  is 
what  happens  when  external  cold  stimulates  all  the  mus- 
cles of  the  body  to  increased  contraction.  Movement  does 
not  usually  result  because  the  extensors  and  flexors  are 
more  or  less  equally  balanced.  There  are  times,  however, 

1  A  muscle  may  contract  without  shortening  or  producing  movement, 
as  when  we  pull  on  a  weight  we  cannot  lift. 


THEEMAL  PHENOMENA  209 

when  this  contraction  of  the  antagonists  is  not  well  bal- 
anced, and  we  "  shiver."  Shivering  is  the  extreme  expres- 
sion of  this  heat-producing  reflex. 

We  are  often  conscious  of  this  increased  muscular  action 
during  cold,  even  when  we  are  not  shivering.  Every  one 
knows  the  difference  between  the  "bracing"  effects  of  a 
cool  or  cold  day  and  the  "  relaxed,"  "  slack-twisted  "  feel- 
ing on  a  warm  day ;  and  this  is  largely  traceable  to  the 
sensations  which  come  from  the  contracting  muscles  in 
the  former  case  and  to  the  absence  of  such  sensations  from 
the  inactive  muscles  in  the  latter.  To  put  it  in  another 
way,  cold  increases  the  tone  of  the  skeletal  muscles  (see 
p.  159).  A  skeletal  muscle  on  a  cold  day  is  never  com- 
pletely relaxed  ;  like  the  unstriped  muscles  of  the  arteries, 
it  is  in  a  condition  somewhere  between  extreme  contraction 
and  extreme  relaxation. 

27.  The  Regulation  of  the  Body  Temperature  a  Function 
of  the  Nervous  System.  —  We  may  close  this  brief  account 
of  thermal  phenomena  of  the  body  by  recalling  to  the 
attention  of  the  student  what  must  now  be  obvious  at  a 
glance,  namely,  that  a  constant  temperature  is  maintained 
by  the  coordinating  action  of  very  many  nervous  reflexes. 
The  action  of  the  vasomotors  of  the  skin  and  of  the 
internal  organs,  of  the  nerves  of  the  sweat  glands  and  of 
the  motor  nerves  of  the  skeletal  muscles,  must  all  be  so 
adjusted  with  regard  to  one  another  that  exactly  the  right 
balance  is  preserved  amid  all  the  variations  of  heat  pro- 
duction and  of  climatic  conditions  which  affect  heat-loss. 
Success  in  this  adjustment  depends  upon  the  skill  with 
which  the  coordinating  nervous  system  does  its  part. 
With  the  single  exception  of  muscular  exertion,  no  con- 
dition of  life  makes  such  far-reaching,  or  such  imperious 
demands  upon  the  system  as  a  whole  as  does  the  mainte- 
nance of  the  proper  internal  temperature.  Mental  work 
and  the  efficiency  of  digestion  are  examples  we  have 


210  THE  HUMAN  MECHANISM 

already  studied  —  and  more  could  easily  be  cited  —  of 
functions  which,  important  as  they  are,  are  subordinated, 
even  sacrificed,  to  prevent  a  marked  rise  or  fall  in  the 
temperature  of  the  blood. 

To  such  an  extent  is  the  nervous  system  as  a  whole 
adapted  to  maintain  the  constant  temperature  that  the 
failure  to  do  this,  as  shown  by  the  presence  of  fever,  or 
by  the  even  more  serious  subnormal  temperature,  becomes 
one  of  the  most  important  indications  that  something  has 
gone  wrong.  We  know  already  how  the  nervous  system 
intervenes  in  every  function  of  our  lives,  and  how  the  well- 
being  of  the  body  as  a  whole  depends  upon  the  adjust- 
ments which  it  brings  about.  It  is  for  these  reasons  that, 
when  it  is  no  longer  able  to  exercise  that  supreme  control 
of  the  constant  temperature  which  is  one  of  its  most  char- 
acteristic features  in  health,  the  physician's  orders  usually 
are  to  "  go  to  bed  and  be  perfectly  quiet."  The  body  is  then 
in  no  condition  to  make  demands  on  the  nervous  system  for 
action ;  and  a  person  who  refuses  to  heed  the  plain  warn- 
ing which  his  temperature  holds  out  has  nothing  but  his 
own  foolishness  to  blame  if  he  suffers  serious  consequences. 


CHAPTER  XIII 
NUTRITION 

A.  THE  SOURCES  OF  POWER  AND  HEAT  FOR  THE 
HUMAN  MECHANISM 

1.  Food  and  Nutrition.  —  The  one  source  of  supply  of 
matter  required  for  the  growth  and  repair  of  the  human 
machinery,  and  of  power  required  for  its  work,  is  food ; 
and  the  study  of  the  various  ways  in  which  different  kinds 
of  food  serve  the  needs  of  the  body,  and  the  value  of  each 
kind  of  food,  under  different  circumstances  or  for  different 
purposes  (such  as  athletic  training,  mental  work,  life  in 
cold  climates,  in  hot  climates,  etc.),  is  the  study  of  nutri- 
tion.   We  shall  limit  our  consideration  of  this  great  subject 
to  a  few  elementary  and  practical  topics,  beginning  with 
the  supply  of  heat  and  power  to  the  body. 

2.  The  Fuel  Value  of  Food.  —  In  any  locomotive  engine 
the  same  amount  of  a  given  fuel  will  enable  the  engine  to 
pull  a  train  of  the  same  weight  for  the  same  distance  over 
the  same  track,  provided,  of  course,  the  engine  itself,  the 
bearings  of  the  wheels,  etc.,  are  in  the  same  condition. 
When  a  ton  of  coal  is  put  into  the  tender,  it  is  with  the 
expectation  that  it  will  move  the  train  a  certain  distance. 
Thus  there  is  a  definite  relation  between  the  fuel  burned 
and  the  work  done.    Every  engineer  knows  also  that  the 
same  weight  of  different  fuels  will  carry  the  train  different 
distances ;  a  thousand  pounds  of  wood,  of  bituminous  coal, 
and  of  anthracite  coal  have  different  fuel  values. 

The  same  weight  of  a  given  fuel  when  burned  will 
always  yield  exactly  the  same  amount  of  heat,  as  is  proved 

211 


212  THE  HUMAN  MECHANISM 

«• 

by  burning  the  fuel  under  conditions  which  enable  us  to 
measure  the  heat  given  off.  The  simplest  means  of  doing 
this  is  perhaps  with  the  ice  calorimeter,  —  a  metal  box 
within  which  the  fuel  is  burned,  the  box  being  every- 
where surrounded  by  a  thick  layer  of  ice.  The  heat  pro- 
duced in  burning  the  fuel  is  measured  by  the  amount  of 
ice  melted. 

In  thi^  way  we  may  find  the  relative  amounts  of  work 
which  can  be  done  with  two  different  fuels ;  for  it  has 
been  discovered  by  actual  experiment  that  if  one  kind  of 
fuel  produces  twice  as  much  heat  as  another,  it  will  also 
do  twice  as  much  work. 

Now  food  is  the  fuel  for  the  muscular  work  of  the  body 
and  also  for  the  liberation  of  heat.  Consequently,  if  we 
determine  how  much  heat  is  liberated  when  a  certain 
amount  of  proteid,  or  fat,  or  carbohydrate  is  burned  in  a 
calorimeter,  we  know  how  much  work  it  may  do  in  the 
body ;  or  at  least  we  know  that  it  can  do  no  more  than  the 
amount  indicated  by  the  calorimetric  experiment. 

3.  Units  of  Heat  and  Work.  —  In  order  to  measure  we 
must  have  units  of  measurement.  Common  units  of  length 
are  the  inch  or  centimeter ;  units  of  area  are  the  square 
yard,  the  square  meter,  or  the  acre ;  units  of  volume,  the 
quart  or  peck;  units  of  weight,  the  pound  or  kilogram. 
And  we  express  the  results  of  these  measurements  by  say- 
ing that  a  thing  is  so  many  inches  long  or  of  so  many 
pounds  weight.  What  are  the  units  of  heat  and  work  ? 

Like  all  units,  these  are  arbitrarily  chosen.  The  unit  of 
heat,  known  as  the  calorie,  is  the  amount  of  heat  necessary 
to  raise  one  gram  (on'e  cubic  centimeter)  of  water  one  degree 
Centigrade.  The  unit  of  work  is  the  amount  of  work  done 
in  lifting  a  kilogram  (2.2  Ibs.)  to  the  height  of  one  meter 
(39.37  in.)  from  the  surface  of  the  earth  against  the  attrac- 
tion of  gravitation.  This  is  known  as  the  Jcilogr  ammeter. 
Thus  when  a  man  weighing  sixty  kilograms  goes  up  a 


NUTRITION  213 

flight  of  stairs  ten  meters  high  his  muscles  do  600  kilo- 
grammeters  of  work.1 

Finally,  it  has  been  found  that  the  same  fuel  which 
when  burned  will  liberate  one  calorie  of  heat  will  supply 
the  power  to  do  0.423985  kilogrammeters  of  work.  By 
this  we  mean  that  not  more  than  0.423985  kilogram- 
meters  of  work  can  be  obtained  from  it.  Not  every  engine 
is  so  perfectly  constructed  as  to  get  from  a  certain  fuel 
its  full  working  capacity ;  indeed,  most  engines  transform 
only  a  small  fraction  of  the  power  of  their  fuel  into  work, 
the  rest  escaping  as  heat,  —  in  the  smoke,  or  by  radiation, 
conduction,  and  convection  from  the  furnace,  boiler,  etc. 
But  by  the  method  above  outlined  it  is  possible  to  find  the 
maximum  amount  of  work  which  can  be  obtained  from  a 
given  weight  of  fuel. 

Applying  the  same  methods  to  food,  we  find  that : 

1  gram  of  dried  proteid  yields  4100  calories. 

1  gram  of  dried  carbohydrate  yields  4100  calories. 

1  gram  of  fat  yields  9300  calories. 

These  figures  are  known  as  the  fuel  values  of  proteids, 
carbohydrates,  and  fats. 

But  the  total  possible  power  which  may  be  obtained  by 
actually  burning  a  certain  substance  under  the  most  favor- 
able conditions  is  one  thing,  and  the  amount  of  power  which 
the  muscles  may  obtain  from  it  is  quite  another.  When  coal 
is  burned  in  an  engine  it  does  work ;  but  the  human  body 
would  get  no  energy  for  its  muscular  work  from  eating 
coal.  So  that  we  have  now  to  inquire  from  what  nutri- 
ents (p.  89)  the  muscles  get  their  energy  for  work,  and  from 
what  nutrients  the  body  derives  its  heat. 

4.  The  Power  for  Muscular  Work.  —  Few  questions  in 
physiology  have  been  more  thoroughly  investigated  than 

1  Work  may  also  be  expressed  in  foot  pounds.  (How  many  foot 
pounds  equal  one  kilogrammeter  ?) 


214  THE  HUMAN  MECHANISM 

this.  In  the  first  half  of  the  nineteenth  century  many 
investigators,  impressed  with  the  fact  that  the  muscle 
fiber  yields,  on  chemical  analysis,  large  quantities  of  proteid 
and  only  traces  of  carbohydrates  and  fats,  believed  that 
the  energy  for  muscular  contraction  comes  entirely  from 
the  consumption  of  the  proteid  of  the  muscle  substance. 
If  this  were  true,  it  would  necessarily  follow  that  proteid 
is  the  proper  food  to  yield  the  energy  for  muscular  con- 
traction, while  fats  and  carbohydrates  would  simply  be 
oxidized  to  give  heat. 

This  view  was  disproved  by  the  following  epoch-making 
experiment  of  physiology.  Two  observers  determined  for 
three  successive  days  the  nitrogen  excreted  by  themselves  ; 
since  almost  all  this  nitrogen  conies  from  proteid,  this  gave 
the  amount  of  proteid  consumed  by  the  body.  On  the  first 
and  third  days  no  vigorous  muscular  work  was  done  ;  on  the 
second  day  they  climbed  a  mountain  1956  meters  (6415  ft.) 
high.  As  one  man  weighed  66  kilograms  and  the  other 
76  kilograms,  the  work  done  in  lifting  the  body  to  the  top 
of  the  mountain  in  the  two  cases  was  129,096  and  148,- 
656  kilogrammeters  respectively.  The  proteid  which  was 
oxidized  in  this  time  could  in  the  two  cases  have  yielded 
power  for  only  68,690  and  68,376  kilogrammeters  of  work. 
In  other  words,  the  proteid  did  not  begin  to  yield  sufficient 
power  for  the  muscular  work  done  in  lifting  the  body  to  the 
top  of  the  mountain  ;  something  else  than  proteid  must  have 
been  oxidized  for  that  purpose,  and  that  something  must 
evidently  have  been  carbohydrate  or  fat,  or  both. 

Again,  it  was  noticed  that  there  was  no  increase  of  pro- 
teid disintegration  on  the  day  of  work ;  this  remained 
practically  unaffected  by  muscular  contraction.  Numerous 
other  experiments  made  since  that  time  have  shown  the 
same  thing.  Muscular  exercise  does  not  necessarily  increase 
proteid  disintegration,  and  the  power  for  it  can  be  obtained, 
in  large  part  at  least,  from  fats  and  carbohydrates. 


NUTRITION  215 

• 

In  the  experiment  above  referred  to  no  determinations 
were  made  of  the  excretion  of  carbon  dioxide.  Since  then 
numerous  experiments  have  been  made  in  which,  on  an 
abundant  mixed  diet,  both  the  nitrogen  and  the  carbon  of 
the  excretions  were  accurately  determined.  These  have 
shown  that  while  muscular  exercise  does  not  necessarily 
increase  proteid  disintegration,  it  invariably  increases  the 
production  of  carbon  dioxide.  If  the  carbon  of  the  carbon 
dioxide  came  from  proteid,  it  would  be  accompanied  by 
increased  excretion  of  nitrogen  derived  from  the  broken- 
down  proteid.  The  fact  that  it  is  not  so  accompanied 
can  only  mean  that  it  came  from  the  oxidation  of  some- 
thing which  did  not  contain  nitrogen,  i.e.  from  fat  or  car- 
bohydrate, or  both. 

But  while  muscular  work  does  not  necessarily  or  even 
usually  increase  proteid  decomposition,  and  the  power  for 
the  same  may  be  derived  largely,  if  not  entirely,  from  car- 
bohydrates and  fats,  it  has  -been  shown  conclusively  that 
under  certain  conditions  this  power  may  come  entirely 
from  proteid.  In  one  experiment  a  large  and  very  lean 
dog  was  fed  for  several  weeks  on  an  abundant  diet  of  lean 
meat,  containing  practically  no  carbohydrate  or  fat ;  dur- 
ing this  time  the  dog  did  large  amounts  of  work  in  a  tread- 
mill and  in  other  ways ;  and  since  it  was  found  that  this 
work  could  be  done  for  weeks  at  a  time  on  the  meat  diet, 
we  conclude  that  the  proteid  must  have  been  the  sole 
source  of  power  for  the  work;  it  must  also  have  served 
as  the  source  of  heat  production,  for  the  normal  tempera- 
ture of  the  animal  was  maintained. 

5.  Summary  of  Considerations  on  the  Supply  of  Power 
for  Work.  —  These  and  other  experiments  show  (1)  that 
the  animal  body  can  get  its  energy  for  mechanical  work 
and  for  the  production  of  heat  from  proteid,  or  from  car- 
bohydrate, or  from  fat  ;  (2)  that  when  the  animal  is  on  an 
abundant  mixed  diet,  even  vigorous  muscular  work  does 


216  THE  HUMAN  MECHANISM 

• 

not  increase  the  oxidation  of  proteid,1  but  that  it  does 
enormously  increase  that  of  carbohydrates  and  fats.  The 
probable  meaning  of  this  is  to  be  sought  in  the  fact  that 
proteid  decomposition  depends  primarily  not  on  muscular 
work  but,  as  we  shall  see  later  (p.  222),  on  the  amount 
of  proteid  eaten ;  while  the  oxidation  of  fats  and  carbohy- 
drates depends  entirely  on  the  demands  of  the  body  for 
energy,  and  is  largely  independent  of  the  amount  of  these 
foods  eaten. 

6.  The  Supply  of  Energy  for  Heat  Production.  "  Heat- 
ing "  Foods.  —  In  studying  the  phenomena  of  heat  pro- 
duction in  the  body  we  found  that  when  the  body  needs 
more  heat  in  order  to  maintain  its  normal  temperature, 
this  heat  is  supplied  chiefly  by  greater  chemical  activity 
in  the  muscles  (p.  207).  The  contraction  or  tone  of  the 
muscles  increases  in  response  to  stimuli  from  the  same 
motor  nerves  which  stimulate  them  to  activity  when  they 
do  external  mechanical  work.  Heat  production  in  the 
body,  from  the  standpoint  of  nutrition,  is  therefore,  as  far 
as  we  know,  largely  a  case  of  increased  muscular  activity; 
and  here,  as  in  the  case  of  mechanical  work,  the  energy 
can  be  obtained  from  one  kind  of  food  as  well  as  from 
another.  Contrary  to  popular  ideas,  we  have  no  conclusive 
evidence  that  one  kind  of  food  supplies  heat  more  readily 
than  another.  What  is  required  in  cold  weather  is  more 
food,  whether  proteid  or  carbohydrate  or  fat.  We  shall 
see  that  there  are  good  reasons  for  not  unduly  increasing 
the  proteid  of  the  diet  under  any  conditions  (p.  225),  and 
hence  in  this  special  case  it  is  probably  better  to  increase 
the  non-nitrogenous  foods  to  a  greater  extent  than  the 
proteids,  though  not  because  they  are  better  "  heating " 
foods. 

1  Under  the  abnormal  conditions  of  excessive  muscular  work  (e.g.  six- 
day  walking  matches  or  bicycle  races)  the  proteid  oxidation  is  often 
increased. 


NUTEITION  217 

There  is  another  reason  why  the  fats  and  carbohydrates 
may  well  be  increased  in  cold  climates,  and  that  is  that 
they  are  par  excellence  the  foods  which  lead  to  the  storage 
of  fat.  The  storage  of  fat  in  the  subcutaneous  tissue,  we 
have  already  seen,  greatly  facilitates  the  maintenance  of 
the  body  temperature  by  interposing  a  poor  conductor 
of  heat  between  the  skin  and  the  more  internal  heat-pro- 
ducing organs;  and  this  gives  to  these  foods  a  special  value 
in  a  cold  climate. 

It  is  often  pointed  out  that  inhabitants  of  cold  climates 
eat  more  fat,  while  the  non-proteid  food  of  those  who  live 
in  warm  climates  consists  chiefly  of  carbohydrates,  and 
the  conclusion  is  drawn  that  fat  is  a  better  food  for  cold 
weather.  This  reasoning,  however,  neglects  the  fact  that 
vegetables,  from  which  alone  carbohydrates  are  obtained, 
are  not  common  in  cold  climates.  The  people  in  such 
climates  get  their  non-proteid  foods  from  the  most  con- 
venient source,  —  the  adipose  tissue  of  animals.  The 
question  is  further  complicated  by  considerations  of  diges- 
tibility of  the  two  kinds  of  food,  and  cannot  be  answered 
offhand. 

B.  SPECIAL  EFFECTS  OF  THE  DIFFERENT  NUTRIENTS 

Our  study  in  the  preceding  section  of  the  sources  of 
power  and  heat  has  led  to  the  conclusion  that  while  one 
nutrient  may  be  preferable  to  another,  either  proteid  or 
carbohydrate  or  fat  may  meet  these  two  needs  of  the  body. 
In  other  words,  no  marked  difference  in  use  has  thus  far 
developed  for  the  oxidizable  nutrients.  In  the  present 
section  we  desire  to  dwell  upon  the  special  uses  of  the 
nutrients,  i.e.  the  uses  which  each  subserves  and  which 
cannot  be  replaced  by  others. 

The  more  important  nutrients  are,  as  we  have  seen  in 
Chapter  VIII,  proteids,  carbohydrates,  fats,  inorganic  salts, 


218  THE  HUMAN  MECHANISM 

and  water.    These  may  now  be  conveniently  classified  as 
follows : 

A.  Compounds  of  carbon  which    f  i.  PROTEIDS 

undergo  marked  chemical 

change,  by  oxidation  or  J  2.  CARBOHYDRATES    ~}       ^^ 
otherwise,  into  waste  prod-  \  proteids 

ucts  within  the  body  [  6-  * ATS  } 

B.  Substances    excreted    from    (  4.  IXORGANIC  SALTS 

the    body  largely  in   the  J 

same  form  as  absorbed      [  5.  WATER  l 

The  proteids  contain  carbon,  hydrogen,  nitrogen,  oxygen, 
and  sulphur ;  some  of  them  also  contain  phosphorus  and 
some  contain  small  quantities  of  iron.  The  carbohydrates 
and  fats  contain  only  carbon,  hydrogen,  and  oxygen.  All 
three  are  oxidized  in  the  body  to  the  waste  products  with 
which  we  have  already  become  acquainted  (see  Chapter  XI). 
The  usual  inorganic  salts  of  the  food,  on  the  other  hand, 
do  not  undergo  oxidation ;  and,  while  they  may  change 
into  other  chemical  forms  during  their  stay  in  the  body,  it 
is  still  true,  on  the  whole,  that  they  are  excreted  in  the 
same  form  as  that  in  which  they  were  absorbed.  And  the 
same  thing  is  true  of  the  water  we  drink. 

7.  The  Albuminoids  and  Extractives.  —  In  addition  to 
the  above,  the  connective  tissues  of  animal  foods  contain  a 
sixth  nutrient  which  is  composed  of  the  same  elements  as 
proteids,  i.e.  carbon,  hydrogen,  nitrogen,  oxygen,  and  some- 
times sulphur.  By  far  the  most  important  member  of  the 
group  is  the  collagen  of  the  fibers  of  connective  tissues, 
bone,  etc.,  and  the  gelatin  into  which  this  collagen  is  con- 
verted when  it  is  boiled  with  water.  These  so-called  albu- 
minoids may  do  the  work  of  proteids  in  part,  but  they 
cannot  replace  the  proteid  entirely. 

Finally,  most  foods  contain  small  quantities  of  other  sub- 
stances, known  as  extractives,  which  are  of  use  to  the  body 

1  See  Chapter  VIII  for  definition  of  the  terms  in  this  table. 


NUTBITION  219 

chiefly  because  they  contribute  to  the  taste  or  flavor  of  the 
food.  Some  of  them  are  thought  to  exert  a  stimulating 
effect,  while  still  others  are  of  importance  in  disease.  For 
our  present  purposes,  however,  they  are  of  minor  impor- 
tance, and  we  shall  confine  our  attention  to  the  nutritive 
value  of  the  proteids,  carbohydrates,  fats,  salts,  and  water. 

8.  The  Inorganic  Salts  and  Water.  —  We  may  begin 
with  these  because,  so  far  as  they  concern  us  in  an  ele- 
mentary work  like  this,  their  uses  are  most  easily  under- 
stood. Inorganic  salts  (chlorides,  sulphates,  carbonates, 
and  phosphates  of  sodium,  potassium,  calcium,  etc.)  are 
found  in  all  living  cells,  in  the  blood  and  the  lymph,  and 
are  constantly  being  removed  from  the  blood  in  the  urine 
and  the  perspiration.  This  loss  must  be  made  good  by  the 
food.  While  inorganic  salts  are  not  oxidized  by  the  body, 
their  presence  in  the  blood  and  tissues  is  for  various  reasons 
absolutely  necessary.  The  craving  of  herbivorous  animals 
for  salt,  in  which  their  food  is  deficient,  is  well  known, 
and  in  parts  of  India  salt  famines  have  occurred  during 
which  the  price  of  salt  was  higher  than  that  of  gold. 

The  loss  of  water  from  the  lungs,  skin,  and  kidneys  is 
made  good  from  two  sources.  In  the  first  place,  proteids, 
carbohydrates,  and  fats  all  contain  hydrogen ;  and  when 
they  are  burned  within  the  body  their  hydrogen  combines 
with  oxygen  to  form  water,  thus : 

C6H12O6  +  6  O2   =  6  CO2  +  6  H2O 
sugar      oxygen      carbon      water 
dioxide 

In  this  way,  on  an  average  diet,  almost  one  pint  of  water 
may  be  produced  within  the  body  in  a  day ;  and  we  are  able  , 
to  understand  how  at  times,  despite  the  constant  loss  of 
water  and  the  comparatively  small  amount  drunk  or  taken 
in  the  food,  the  tissues  do  not  dry  up.  Usually,  however, 
this  supply  is  insufficient,  and  it  is  necessary  to  use  water 
as  an  article  of  food. 


220  THE  HUMAN  MECHANISM 

9.  The  Proteid  and  the  Non-Proteid  Foods :  Three  Points 
of  Difference.  —  We  may  pass  now  to  the  study  of  those 
food  stuffs  which  are  compounds  of  carbon,  and  which 
undergo   marked   chemical    change   into    waste   products 
within  the  body.    We  have  already  separated  those  food 
stuffs  —  proteids,  carbohydrates,  and  fats  —  into  two  divi- 
sions, the  proteids  and  the  non-proteids,  because  in  at  least 
three  respects  the    members   of   the  two    divisions  play 
different  r61es  in  the  nutrition  of  the  body.    They  differ  (1) 
in  their  availability  to  serve  in  repairing  losses  of  living 
tissue  ;  (2)  in  their  effects  upon  the  chemical  changes  in 
the  body  ;  and  (3)  in  their  relation  to  the  storage  of  fat.    In 
all  these  respects  the  fats  and  carbohydrates  resemble  each 
other  closely,  while  they  differ  from  the  proteids. 

10.  The  Waste  and  Repair  of  the  Living  Cells.  —  Certain 
organs  of  the  body,  notably  the  muscles  and  glands,  con- 
sist chiefly  of  living  cells,  thus  presenting  a  striking  con- 
trast to  other  organs,  like  bones,  tendons,  ligaments,  and 
cartilage,  in  which  the  living  cells  are  few  in  number,  the 
main  mass  of  the  organ  consisting  of  lifeless  matter  between 
the  cells.    If,  therefore,  we  make  a  chemical  analysis  of 
organs  of  the  first  kind,  we  can  obtain  some  insight  into  the 
composition  of  the  living  cell.    The  results  of  the  analysis 
of  muscle  (lean  meat)  will  serve  as  a  type  of  what  is  found 
in  such  cases. 

Composition  of  lean  meat : 

Water 75% 

Solids 25% 

Proteid 21% 

Salts 1% 

Extractives 1% 

Fat,  connective  tissue,  etc 2% 

We  do  not  know  in  what  form  the  proteid  which  we 
find  in  dead  muscle  is  present  in  living  muscle ;  but  even 
if  it  is  not  in  the  precise  form  which  we  know  as  lifeless 


NUTRITION  221 

proteid  (such  as  the  white  of  an  egg),  it  is  still  true  that 
this  proteid  constitutes  the  chemical  basis  of  the  living 
substance.  Hence  we  may  say  that  the  living  cell  is 
largely  composed  of  a  proteid-like  substance  containing 
among  other  elements  carbon,  hydrogen,  nitrogen,  oxy- 
gen, and  sulphur. 

11.  The  Proteid-like  Substances  of  the  Living  Cell  are 
constantly   disintegrating.  —  Actual  study  of   the  excre- 
tions of  the  human  body  shows  that  under  all  conditions 
of  its  life  nitrogenous  substances  and  sulphates  are  given 
off.    Since  the  nitrogen  and  sulphur  which  these  contain 
are  found  only  in  the  proteids  of  the  food,  it  follows  that 
there  is  a  constant  disintegration  of  proteid  going  on  in 
the  body.    This  is  observed  during  fasting  and  even  when 
an  abundance  of  fats,  carbohydrates,  salts,  and  water,  but 
no  proteid,  is  taken  in  the  food.    The  disintegration  of 
proteid  seems  to  be  as  truly  a  part  of  the  life  of  the  body 
and  of  its  constituent  cells  as  is  the  consumption  of  oxy- 
gen ;  and  all  the  facts  show  that  whether  we  are  doing 
muscular  work  or  mental  work,  whether  we  are  asleep  or 
awake,  whether  the  weather  be  warm  or  cold,  in  short,  no 
matter  what  may  be  the  conditions  of  life,  this  disintegra- 
tion of  proteid  is  going  on  in  every  living  cell.    We  shall 
subsequently  learn  that  it  is  greater  at  one  time  than  at 
another,  but  that  it  never  ceases.    Hence  it  fojlows  that 
proteid  food,  which  alone  contains  the  nitrogen  and  sul- 
phur needed  to  make  good  this  loss,  is  an  indispensable 
part  of  the  diet ;  and  it  has  been  shown  that  when  their 
diet  contains  no  proteid,  animals  starve  as  surely  as  they  do 
when  they  eat  no  food  at  all.    Fats  and  carbohydrates,  on 
the  other  hand,  may  be  excluded  from  the  diet  without 
fatal,  and  sometimes  without  harmful  results. 

12.  The  Effect  of  Proteids  and  Non-Proteids  on  the  Con- 
sumption of  Material  in  the  Body The  amount  of  coal  or 

other  fuel  burned  in  an  open  fire  depends  to  a  large  extent 


222  THE  HUMAN  MECHANISM 

on  the  amount  put  on  the  fire,  and  is  independent  of  the 
need  for  heat  in  the  room.  Is  this  true  of  the  food  taken 
into  the  body? 

Actual  study  of  the  subject  has  shown  that  the  proteid 
and  the  non-proteid  foods  differ  greatly  in  this  respect. 
When  more  proteid  is  eaten,  digested,  and  absorbed,  more 
proteid  is  consumed  in  the  body  and  more  waste  products 
produced.  Here  there  is  some  resemblance  to  an  open  fire. 
Feeding  more  of  proteid  does  actually  lead  to  greater  oxi- 
dation of  proteid  in  the  body. 

With  the  non-proteid  nutrients  this  is  not  true,  or  at 
least  not  to  the  same  extent.  The  body  uses  only  as  much 
of  them  as  its  needs  require,  or  but  little  more  than  this, 
and  stores  the  excess  for  future  use.  It  behaves  toward 
them  somewhat  as  the  fire  would  behave  toward  the  fuel 
if,  independently  of  the  amount  of  fuel  fed,  it  burned 
only  so  much  as  was  necessary  to  heat  the  room  to  the 
proper  temperature.  The  behavior  of  the  body  toward 
them  suggests  the  action  of  the  automatic  regulator  which 
closes  the  draft  of  a  furnace  as  the  temperature  of  the 
room  rises,  and  so  limits  the  consumption  of  fuel  according 
to  the  need  for  heat.  The  body,  then,  behaves  toward  pro- 
teid somewhat  as  an  open  fire  does  toward  the  fuel  put 
upon  it,  while  in  its  behavior  toward  fats  and  carbohydrates 
it  is  more, like  a  self-regulating  furnace. 

13.  The  Relation  of  Proteid  Feeding  to  the  Quantity  of 
Proteid-like  Substance  in  the  Living  Cells  of  the  Body. —  We 
have  shown  in  the  last  paragraph  that  when  more  proteid 
is  eaten  more  is  consumed  in  the  body.  We  may  now  point 
out  another  important  relation  of  proteid  to  nutrition.  It 
has  been  found  that  if  a  man,  whose  food  meets  all  require- 
ments of  his  body  so  that  he  is  in  good  health,  does  full 
work,  and  is  neither  gaining  nor  losing  in  weight,  increases 
the  amount  of  proteid  in  his  food,  not  all  the  extra  proteid 
is  disintegrated  into  waste  products,  but  that  some  of  it  is 


NUTBITION  223 

retained  in  the  body,  apparently  increasing  the  total  amount 
of  living  cell  substance.  If  now  the  increased  proteid  diet 
be  continued  from  day  to  day,  it  will  be  found  that  the 
amount  of  proteid  thus  reserved  or  the  amount  of  living 
cell  substance  thus  gained  becomes  less  from  day  to  day, 
until  finally  the  amount  of  proteid  disintegrated  exactly 
equals  that  absorbed.  The  condition  thus  finally  attained 
is  known  as  nitrogenous  equilibrium,  which  means  that  the 
nitrogen  of  the  intake  (proteid  food)  exactly  equals  the 
nitrogen  of  the  output  (urea,  etc.);  and  so  long  as  the  same 
amount  of  proteid  is  fed  this  state  of  nitrogenous  equilib- 
rium continues.  Such  experiments,  therefore,  prove  that 
increasing  the  proteid  of  the  diet  not  only  leads  to  increased 
proteid  consumption  but  also  to  an  increase  of  the  stock 
of  proteid-like  material  within  the  cells  of  the  body. 

Furthermore,  if  after  the  establishment  of  nitrogenous 
equilibrium  the  amount  of  proteid  in  the  diet  be  decreased, 
the  body  for  a  time  excretes  more  nitrogen  than  it  takes 
in ;  that  is  to  say,  it  consumes  all  the  proteid  of  the  food 
and,  in  addition,  some  of  the  proteid-like  substance  of  its 
own  living  cells.  Obviously  the  amount  of  proteid  within 
the  living  cells  of  the  body  depends  on  the  amount  of  pro- 
teid absorbed  from  the  alimentary  canal.  We  may,  there- 
fore, conclude  that  (1)  the  amount  of  proteid  digested  and 
absorbed  is  a  determining  factor  in  the  amount  of  proteid 
consumed  or  disintegrated  in  the  body ;  and  (2)  it  is  a  deter- 
mining factor  in  the  proteid  contained  within  the  body. 
In  other  words,  to  maintain  a  given  amount  of  proteid  in 
the  living  cells,  a  certain  amount  of  proteid  is  necessary 
in  the  food;  less  than  this  will  lead  to  loss  of  proteid, 
more  will  increase  it. 

With  the  non-proteid  foods  the  case  is  different.  Eating 
more  fat  or  carbohydrate  does  not  necessarily  change  the 
amount  of  these  substances  consumed  or  oxidized.  The 
body  seems,  on  the  whole,  to  use  as  much  of  them  as  it 


224  THE  HUMAN  MECHANISM 

needs  (see  pp.  213-217),  and  to  store  up  the  rest  chiefly 
as  fat.  Here  then  is  the  second  point  of  difference  between 
proteid  and  non-proteid  foods  ;  the  amount  of  the  former 
which  the  'body  disintegrates  varies  conspicuously  with 
the  amount  fed,  while  the  disintegration  of  the  latter  is 
chiefly  determined  by  other  causes. 

14.  The  Effects  of  Proteids  and  Non-Proteids  respectively 
on  the  Storage  of  Fat  within  the  Body.  —  The  third  point 
of  difference  between  proteid  and  non-proteid  food  is  that 
the  latter  readily  lead  to  the  storage  of  fat,  —  in  other 
words,  that  they  are  "fattening  foods";  while  the  former 
are  converted  into  fat  only  to  a  much  smaller  extent,  and 
normally  are  possibly  not  turned  into  fat  at  all.   This  sub- 
ject will  be  more  fully  discussed  in  the  next  chapter. 

15.  Summary.  —  In  three  respects,  then,  the  proteid  and 
non-proteid  foods  exert  different  nutritional  effects  upon 
the  body :  first,  only  the  proteids  can  repair  waste  of  liv- 
ing tissue ;   second,  the  mere  act  of  eating  more  proteid 
leads  to  increased  proteid  disintegration  and  to  increased 
chemical  change  in  the  body;  while  the  excess  of  non- 
proteids  over  arid  above  the  needs  of  the  body  for  power 
and  heat  is  stored  away  for  future  use ;  and  third,  fat  is 
formed  chiefly,  if  not  entirely,  from  the  non-proteid  foods. 

On  the  other  hand,  many  requirements  of  the  body  may 
be  met  by  either  class  of  food  stuffs,  though  one  may  do 
better  than  the  other.  In  the  previous  chapter  we  found, 
for  example,  that  both  proteids  and  non-proteids  may  sup- 
ply power  and  heat.  In  some  respects  it  is  a  matter  of 
indifference  whether  we  eat  proteid  or  carbohydrate  or  fat ; 
in  other  respects  it  is  a  matter  of  considerable  importance. 

16.  Other  Considerations  with  Regard  to  Special  Effects 
of  Proteids  and  Non-Proteids. — We  have  repeatedly  spoken 
above  of  the  amount  of  a  nutrient  "eaten,  digested,  and 
absorbed,"  instead  of  merely  the  amount  "eaten."    This 
was  done  to  emphasize  the  fact  that  the  nutritional  effects 


NUTRITION  225 

we  have  described  are  produced  by  the  material  which 
passes  into  the  blood  by  absorption,  and  not  merely  by  that 
which  is  swallowed.  For,  in  the  first  place,  a  part  of  the 
food  eaten  is  not  digested  at  all,  but  leaves  the  body  with 
the  bowel  discharges ;  the  amount  of  food  thus  lost  to 
the  organism  varies  with  different  individuals.  Observa- 
tions are  on  record  which  show  that  at  times  as  much  as 
one  third  of  the  proteid  eaten  thus  escapes  the  action  of 
the  digestive  juices.  Nor  is  this  all.  We  have  seen  that 
more  or  less  of  the  food  eaten  is  consumed  within  the  in- 
testine, and  sometimes  even  within  the  stomach,  by  bac- 
teria. Proteids,  carbohydrates,  and  fats  thus  destroyed  by 
bacterial  action  produce  none  of  the  effects  described  above. 
These  effects  are  produced  only  by  those  foods  which  are 
digested  and  absorbed  into  the  blood. 

These  considerations  partly  explain  the  striking  fact 
that  some  people  remain  lean  although  they  are  "great 
eaters,"  while  others  grow  fat  on  a  comparatively  meager 
diet.  In  other  words,  it  is  one  thing  to  take  food  into  the 
alimentary  canal  and  quite  another  thing  to  digest,  absorb, 
and  in  acceptable  form  distribute  it  to  the  living  cells  of 
the  body. 

Another  point  is  of  very  considerable  practical  impor- 
tance and  should  be  reckoned  with  in  the  choice  of  food. 
The  carbon  dioxide  and  water  into  which  the  fats  and  car- 
bohydrates are  oxidized  in  the  cells  are  at  once  taken  up 
by  the  blood  and  carried  directly  to  the  lungs  and  other 
excretory  organs  where  they  are  got  rid  of.  With  the 
waste  products  of  proteid  disintegration  the  case  is  differ- 
ent. Proteid  does  not  break  down  in  the  cells  at  once  into 
urea,  etc.,  but  largely  into  intermediate  products ;  these 
are  thereupon  taken  to  other  organs,  chiefly  the  liver, 
where  they  are  chemically  changed  into  urea;  and  it  is 
only  after  this  series  of  chemical  changes  that  the  proteid 
waste  is  carried  in  its  final  form  to  the  kidneys  for  removal 


226  THE  HUMAN  MECHANISM 

from  the  body.  Evidently  it  is  a  more  serious  task  to  elimi- 
nate the  waste  products  of  proteids  than  those  of  fats  and 
carbohydrates.  While  the  body  is  no  doubt  quite  capable 
of  caring  for  large  amounts  of  these  proteid  wastes,  there 
are  reasons  for  thinking  that  there  is  such  a  thing  as 
overburdening  the  liver  and  even  the  kidneys  with  them. 
Consequently,  although  proteids  yield  heat  and  the  power 
for  muscular  work  as  readily  as  do  carbohydrates,  it  may 
be  better  to  use  carbohydrates  for  these  purposes  rather 
than  proteids. 

Finally,  questions  of  digestibility  often  complicate  the 
problem  of  meeting  the  nutritional  demands  of  the  body. 
It  may  well  be  that  of  two  diets  we  may  choose  one  rather 
than  the  other  because  it  is  more  digestible,  and  not  because, 
when  digested,  its  constituents  more  perfectly  meet  the 
demands  of  the  body.  Thus  100  grams  of  proteid  4- 100 
grams  of  fat  -f  300  grams  of  carbohydrate  have  together  the 
same  approximate  nutritional  value  as  100  grams  of  pro- 
teid -f  50  grams  of  fat  +  415  grams  of  carbohydrate  (since 
50  grams  of  fat  are  equivalent  to  115  grams  of  carbo- 
hydrate) ;  but  a  man  whose  digestive  system  cares  for  fat 
with  difficulty  might  well  choose  the  latter  diet  rather  than 
the  former. 

17.  Are  there  Special  Foods  for  Special  Organs  or  for 
Special  Work?  —  This  question  is  raised  by  the  flaming 
advertisements  of  "  nerve "  foods,  "  brain  "  foods,  foods 
ugood  for  muscle,"  or  for  other  parts  of  the  body,  which 
are  often  seen  in  public  journals  or  public  places.  The  stu- 
dent who  has  mastered  the  foregoing  chapters  of  this  book 
hardly  needs  to  be  told  that  in  the  swiftly  circulating  blood 
all  cells  of  all  tissues  have  a  common  ration  and  practically 
sit  at  one  table,  very  much  as  officers  and  soldiers  do  while 
engaged  in  actual  warfare.  No  one  then  thinks  of  giving  to 
officers  brain  food  and  to  common  soldiers  muscle  food,  but 
rather  to  both  a  good,  general,  "  all-around  "  food  supply. 


NUTRITION  227 

Moreover,  any  one  who  will  recall  the  complex  chemical 
processes  of  digestion  cannot  have  forgotten  that  all  foods 
are  greatly  changed  during  that  process,  so  that  if  a  special 
food  is  eaten,  there  is  no  evidence  that  it  ever  reaches  any 
special  cells,  such  as  those  of  brain  or  muscle,  in  or  even 
near  its  original  form  ;  or,  if  it  should  reach  the  desired 
organ  or  tissue,  that  it  would  be  either  wanted  or  used 
there.  Very  likely  some  persons  sometimes  take  special 
foods  or  drinks  with  apparent  or  temporary  advantage,  due 
to  change  of  diet  or  to  a  real  increase  of  food  supply ;  but 
that  their  improved  feeling  or  condition  is  due  to  any  special 
feeding  of  brain  or  muscle  or  other  tissue  is  unlikely ;  and 
the  same  or  even  better  effect  could  probably  have  been 
obtained  by  the  same  amount  of  attention  paid  to  a  change 
of  diet,  increased  muscular  activity,  rest,  sleep,  or  any  one 
of  a  number  of  the  factors  of  hygienic  living. 

C.  FLESH,  FAT,  AND  GLYCOGEN 

18.  Flesh  and  Fat. — People  of  the  same  height  differ 
greatly  in  weight.  We  recognize  this  fact  in  the  use  of  the 
adjectives  "stout"  and  "thin,"  or  "lean."  We  also  recog- 
nize the  fact  that  a  man  of  heavy  build  may  owe  his  weight 
to  fat  or  to  muscle.  Sometimes  we  hear  a  man  say,  "  I  weigh 
the  same  as  formerly,  but  I  have  got  less  muscle  and  more 
fat,"  or  vice  versa.  This  expresses  a  physiological  distinc- 
tion which  is  not  always  made  in  our  common  use  of  words; 
for  when  we  say  some  one  is  "stout"  or  "fleshy"  or  "cor- 
pulent," we  do  not  state  whether  his  greater  weight  is  due 
to  muscle  or  to  fat,  although  we  more  commonly  mean  that 
it  is  due  to  fat. 

It  will  be  convenient  for  our  purposes  if  we  agree  upon 
a  definite  use  of  terms  to  express  this  difference.  In  a 
piece  of  beefsteak  there  are  three  well-known  parts,  —  the 
lean  meat,  the  fat,  and  the  connective  tissue;  and  this 


228  THE  HUMAN  MECHANISM 

composition  of  one  of  the  organs  of  the  body  corresponds 
to  the  composition  of  the  body  as  a  whole,  which  consists 
of  three  kinds  of  material :  (1)  the  living  cells,  represented 
by  the  muscle  fibers,  gland  cells,  nerve  cells,  etc.;  (2)  the 
connective  and  supporting  tissues,  such  as  tendon,  liga- 
ment, bone,  and  cartilage ;  and  (3),  stored  away  within  cer- 
tain cells,  lifeless  matter  or  food  material,  which  may  be 
called  upon  to  supply  in  part  the  needs  of  the  body  when 
food  is  not  being  absorbed  from  the  alimentary  canal.  Fat 
is  one  of  the  two  most  important  members  of  the  third 
class. 

With  the  second  of  the  above  classes  (connective  and 
supporting  tissues)  we  are  not  concerned  in  the  present 
chapter,  since  they  are  not  rapidly  consumed  in  the  work 
of  the  body  and  hence  do  not  suffer  large  loss  of  substance, 
which  must  be  made  good  from  the  food.  With  the  first 
and  third  groups,  however,  we  are  immediately  concerned, 
since  they  stand  in  intimate  relation  to  the  cellular  activi- 
ties, and  so  are  subject  to  chemical  change. 

To  the  essential  living  substance  of  the  cells,  which 
we  have  seen  is  built  chiefly  from  proteid,  we  give  the 
name  flesli,  and  it  is  only  with  this  meaning  that  we  shall 
subsequently  use  the  term.  Flesh,  then,  would  include 
the  living  substance  of  the  muscle  fibers,  of  the  cells  of  the 
pancreas,  kidney,  liver,  nervous  system,  etc. ;  in  short,  all 
the  material  of  the  cell  which  is  essential  to  its  life.  It 
does  not  include  the  lifeless  matter  which  may  be  stored 
away  within  these  cells.  Thus  a  cell  of  the  liver  may  con- 
tain drops  of  lifeless  fat  in  addition  to  its  living  flesh. 

19.  The  Increase  of  Flesh.— The  fat  and  flesh  of  the 
body  may  vary  in  amount  at  different  times;  one  of  them 
may  change  while  the  other  remains  constant,  or  both  may 
change  simultaneously.  The  amount  of  each  is,  to  a  large 
extent,  independent  of  that  of  the  other;  and  we  have 
now  to  inquire  what  is  known  about  the  conditions  which 


NUTRITION  229 

determine  how  much  flesh  and  how  much  fat  the  body 
contains  at  any  given  time. 

We  have  already  described  in  the  preceding  chapter  one 
of  the  conditions  of  the  storage  of  flesh  in  the  body,  namely 
abundant  proteid  feeding.  Simply  feeding  more  proteid 
certainly  leads  at  times  to  the  formation  of  more  flesh. 
The  student  will  review  what  has  been  said  on  page  223. 
But  it  is  by  no  means  certain  that  this  is  the  only  condi- 
tion that  leads  to  increase  of  flesh.  The  growth  of  muscles 
by  use  suggests  that  under  conditions  of  active  muscular 
work  more  of  the  proteid  eaten  is  retained  in  the  body  as 
flesh,  and  less  broken  down  to  waste  products,  than  when 
little  muscular  work  is  done.  The  whole  subject  needs 
further  elucidation. 

20.  The  Hoarding  of  Inactive  or  Lifeless  Food  Material. 
The  Storage  of  Fat.  —  The  body  not  only  adds  from  time 
to  time  to  its  stock  of  living  flesh,  but  also  stores  lifeless 
material  for  future  use,  and  that,  for  the  greater  part,  in 
two  forms,  — fat  and  glycogen.  Fat  may  be  stored  as  drops 
of  oil  in  the  cytoplasm  of  any  cell  of  the  body.  Muscle 
fibers,  for  example,  contain  at  times  large  quantities  of 
this  substance,  and  are  then  said  to  have  undergone  fatty 
degeneration.  Under  normal  conditions,  however,  the  pres- 
ence of  considerable  quantities  of  fat  in  muscle  fibers  or 
nerve  cells  or  most  gland  cells  is  unusual.  In  the  cells  of 
connective  tissue,  on  the  other  hand,  fat  is  readily  stored 
under  normal  conditions,  and  the  adipose  tissue  or  fat  of 
the  body  is  simply  connective  tissue  whose  cells  are  loaded 
with  droplets  of  fat.  Figs.  83-85,  with  their  explanation, 
will  show  how  this  takes  place.  But  while  fat  may  be  nor- 
mally stored  in  any  of  the  more  open  connective  tissues,  it 
is  especially  in  the  subcutaneous  tissue,  the  great  omentum, 
the  mesentery,  and  some  other  situations  that  its  chief 
storage  takes  place.  From  these  storehouses  it  is  drawn 
upon  as  a  reserve  food  material  when  the  immediate  supply 


230 


THE  HUMAN  MECHANISM 


Flo.  83 


FIG.  84 


FIG.  85 


FIGS.  83-85.  Three  successive  stages  in  the 
transformation  of  ordinary  connective 
tissue  into  adipose  tissue 

A  portion  of  the  capillary  network  is  shown, 


of  food  from  the  alimentary 
canal  becomes  inadequate  for 
the  work  of  the  body.  The 
exact  mechanism  by  which  it 
is  stored  in  a  cell  at  one  time 
and  discharged  at  another  is 
not  fully  understood.  Some 
of  the  conditions  under 
which  it  is  accumu- 
lated, and  some  of  those 
under  which  it  disap- 
pears, are  known ;  but 
we  do  not  know  the 
whole  story.  Some  peo- 
ple lay  up  fat  in  larger 
quantities  than  others 
on  the  same  diet,  and 
apparently  while  doing 
the  same  amount  of 
work;  and  some  keep 
lean  under  conditions 
apparently  the  most 
favorable  for  growing 
fat. 

It  was  formerly  be- 
lieved, and  is  still  some- 
times supposed,  that 
the  animal  body  forms 


fat  only  from  the  fat 
of  the  food  ;  that  to  get 
fat  we  must  eat  fat. 


surrounded  hy  the  fibers,  among  which  are  This  Was  disproved  by 
several  cells.    The   accumulation  of   fat  "  , 

droplets  in  the  cell  cytoplasm  is  shown  in  a    number    OI    experi- 

Fig.  84,  and  the  fusion  of  these  to  form  ments,    especially  one 

one  large  droplet,  surrounded  hy  the  cyto-  ,  .  .         , 

plasm,  is  seen  in  Fig.  85  by     Lieblg,    who     kept 


NUTEITION  231 

account  over  a  long  period  of  the  fat  in  the  food  sup- 
plied to  a  cow,  and  found  that  the  fat  given  off  in  the 
cow's  milk  far  exceeded  that  in  her  food.  In  another 
experiment  four  pigs  out  of  a  litter  of  eight  were  killed 
and  the  total  amount  of  fat  in  their  bodies  determined. 
The  other  four  pigs  were  fattened  for  a  time,  then  killed, 
and  the  fat  in  their  bodies  eventually  determined.  Assum- 
ing that  the  second  set  of  four  pigs  originally  had  the 
same  quantity  of  fat  as  the  first  four,  the  difference  between 
the  two  quantities  of  fat  found  would  give  the  quantity  of 
fat  the  last  four  had  stored  up.  Meantime,  strict  account 
had  been  kept  of  the  fat  supplied  in  the  food  of  the  last 
four,  and  it  was  shown  that  for  every  100  parts  of  fat  fed 
to  them  these  pigs  had  laid  up  472  parts  of  fat.  They  had 
evidently  manufactured  fat  from  some  substance  other  than 
the  fat  in  their  food. 

21.  Fats  can  be  stored  from  Fats  and  Carbohydrates  in 

Food There  is  no  doubt  that  fat  may  be  both  stored 

away  and  manufactured  from  the  fats  in  the  food.    There 
is  also  no  doubt  that  large  quantities  of  fat  may  be  and 
often  are  manufactured  and  stored  from  the  carbohydrates 
(sugars,  starches,  etc.)  of  the  food ;  so  that,  while  there  is 
some  truth  in  the  idea  that  one  may  get  fat  by  eating  fat, 
it  is  equally  true  that  we  can  get  fat  by  eating  other  foods. 

22.  Are  Proteids  a  Source  of  Fat? — Whether  fats  are 
normally  made  in  the  body  from  proteids  is  a  more  diffi- 
cult question.    There  is  no  undisputed  case  on  record  of 
such  manufacture  and  storage ;  and  while  the  facts  do  not 
yet  justify  us  in  denying  the  possibility,  there  can  be  little 
doubt  that  such  transformation  does  not  take  place  to  any 
great  extent,  and  it  is  possible  that  in  the  mammalian  body 
it  does  not  normally  occur  at  all. 

Fats,  then,  are  manufactured  readily  from  fats  and  car- 
bohydrates, and  sparingly,  if  at  all,  from  proteids.  Their 
disappearance  during  starvation,  when  they  are  drawn 


232  THE  HUMAN  MECHANISM 

upon  to  supply  power  and  heat  for  the  body,  shows  that 
they  serve  as  a  true  reserve  food  material.  They  are  a 
kind  of  food  capital  or  hoard,  saved  and  laid  up  by  the 
body  against  a  rainy  day. 

23.  The  Hoarding  of  Inactive  or  Lifeless  Food  Material. 
The  Storage  of  Glycogen.  —  In  many  cells  of  the  body,  but 
especially  in  those  of  the  liver  and  to  a  less  extent  in  those 
of  the  skeletal  muscles,  there  is  found  a  carbohydrate  sub- 
stance known  as  glycogen.  This  substance  belongs  to  the 
same  group  of  carbohydrates  as  starch  and  dextrin es  (see 
Chapter  VIII),  and  is  sometimes  called  animal  starch.  Like 
them  it  is  changed  into  sugar  by  the  action  of  saliva  and 
pancreatic  juice,  whence  its  name  (V\VKVS  sweet;  -76^9, 
former).  The  same  change  occurs  on  the  death  of  the 
cells  in  which  it  is  contained,  the  sugar  thus  formed  giv- 
ing to  such  tissues  a  sweetish  taste.  This  is  often  noticed, 
for  example,  in  liver  and  in  scallops  (the  shell  muscle 
of  Pecten).  The  total  amount  of  glycogen  in  the  human 
body  may  reach  100-200  grams  (3-7  ounces),  one  half 
of  which  is  concentrated  in  the  liver  and  the  other  half 
scattered  about  in  the  other  tissues  of  the  body. 

Experiments  have  shown  that  glycogen  is  not  formed 
from  the  fat  in  food ;  that  it  is  formed  in  small  quantities 
from  proteid ;  while  its  chief  source  is  the  carbohydrates  of 
the  food. 

The  blood  may  be  said  to  be  always  sweet,  its  constant 
percentage  of  sugar  (2-3  grams  per  1000  cubic  centimeters 
of  blood  plasma)  being  a  striking  fact,  and  one  that  we 
should  hardly  have  anticipated.  One  might  suppose  that 
the  sugar  in  the  blood  would  increase,  as  does  the  amount  of 
fat,  during  active  digestion  and  absorption ;  and  that,  after 
digestion  had  ended,  it  would  diminish.  As  a  matter  of  fact 
the  amount  of  sugar  in  the  blood  remains  practically  constant 
for  many  hours  after  the  completion  of  digestion,  and  this 
despite  the  fact  that  the  tissues  are  constantly  abstracting 


NUTRITION  233 

sugar  from  the  blood.  Evidently  the  blood  must  be  supplied 
with  sugar  from  some  other  source  than  the  alimentary 
canal,  and  there  must  be  somewhere  in  the  body  a  compen- 
sating mechanism  controlling  the  sugar  supply  of  the  blood. 

Experiments  have  shown  that  sugar  is  absorbed  from 
the  alimentary  canal  entirely  by  the  intestinal  blood  ves- 
sels. It  must  pass,  therefore,  through  the  liver  by  the 
portal  vein  (see  Fig.  58,  p.  125)  before  going  to  the  rest 
of  the  body.  The  liver,  thus  standing  at  this  great  gate- 
way to  the  circulation,  would  seem  to  act  as  the  carbo- 
hydrate storehouse,  or  savings  bank,  of  the  body.  Any 
excess  of  sugar  in  the  portal  blood  is  there  transformed 
into  glycogen  and  deposited,  saved  until  it  is  needed,  and 
then  "paid  out,"  as  sugar,  when  the  ready  supply  is  ex- 
hausted. Other  tissues  doubtless  aid  in  preventing  an 
undue  richness  of  sugar  in  the  blood,  acting  likewise  as 
temporary  storehouses  for  this  form  of  food.  If  it  were 
not  for  some  such  provision  as  these  together  make,  the 
eating  of  a  quantity  of  candy  or  of  maple  sugar  might 
easily  add  to  the  blood  enough  sugar  to  damage  the  tissues. 

The  body  is  thus  not  immediately  dependent  upon  the 
food  which  is  being  absorbed  from  the  alimentary  canal  at 
the  time.  In  the  glycogen  and  fat  it  has  within  itself  a 
stock  of  hoarded  non-proteid  material  upon  which  it  may 
call  to  meet  its  requirements  for  power  and  heat,  and  by 
which  it  is  protected  against  temporary  shortage  of  these 
nutrients  in  the  diet.  When,  on  the  other  hand,  the  pro- 
teid  of  the  diet  is  lacking,  the  body  can  make  good  the 
deficit  only  by  yielding  up  its  own  flesh. 

D.  THE  CHOICE  OF  FOOD  AND  NUTRIENTS 

When  we  buy  food  for  the  table  we  do  not,  of  course, 
ask  for  proteids,  carbohydrates,  fats,  salts,  etc.  Foods  sold 
in  markets  are  mixtures  of  these  nutrients  in  varying 


234  THE  HUMAN  MECHANISM 

proportions.  Moreover,  most  meals  consist  of  mixtures  of 
several  foods,  and  the  foods  used  at  one  meal  differ  from 
those  used  at  another,  so  that  the  diet  becomes  what  is 
known  as  a  "  mixed  diet."  In  this  way  two  results  are 
secured :  first,  the  food  is  rendered  more  appetizing,  —  an 
important  physiological  and  hygienic  consideration ;  and, 
second,  we  avoid  the  undue  preponderance  of  one  nutrient 
over  another,  which  would  almost  surely  follow  the  use  of 
a  single  kind  of  food.  Lean  meat,  for  example,  contains  a 
large  amount  of  proteid  and  little  fat  or  carbohydrate ; 
potatoes,  on  the  other  hand,  contain  little  proteid,  but  a 
large  amount  of  carbohydrate ;  by  using  the  two  at  the 
same  meal  we  obtain  better  proportions  of  proteid  and 
carbohydrate;  and  if  butter  be  used  on  the  potatoes,  we 
add  fat  to  the  dietary. 

These  three  nutrients  play  different  r61es  in  nutrition, 
and  it  is  not  a  matter  of  indifference  whether  any  one  of 
them  is  absent,  deficient,  or  present  in  excess.  In  other 
words,  while  the  proportions  of  the  different  nutrients  is 
not  the  chief  consideration  in  the  selection  of  food,  —  such 
things  as  digestibility,  flavor,  bulk,  amount  of  indigestible 
material,  etc.,  also  entering  into  the  decision  of  the  ques- 
tion, —  it  is  nevertheless  an  important  consideration,  and 
there  can  be  no  doubt  that  some  persons  are  poorly  nour- 
ished because  they  err  in  this  respect.  We  have,  therefore, 
to  consider  when  we  may  need  more  proteid,  or  more  fat,  or 
more  carbohydrate  in  the  food  in  order  to  meet  properly 
the  different  conditions  of  daily  life,  such  as  prolonged  or 
hard  muscular  work,  exposure  to  cold  or  warm  weather, 
mental  work,  and  so  on. 

24.  The  Effect  of  Occupations  and  of  External  Conditions 
upon  the  Consumption  of  Food.  —  Since  it  is  the  function 
of  food  to  make  good  the  material  losses  of  the  body,  we 
naturally  ask,  first  of  all,  how  the  various  occupations  and 
conditions  of  life  influence  the  excretions,  for  this  would 


NUTRITION  235 

give  us  some  information  as  to  the  material  whose  loss 
must  be  made  good.  Many  experiments  have  been  made 
to  determine  this  ;  but  their  results  may  be  briefly  sum- 
marized by  saying  that  only  three  things  seem  to  affect 
directly  the  consumption  of  food  in  the  body,  viz.  mus- 
cular activity,  changes  of  temperature,  and  the  act  of  feeding 
itself.  The  changes  thus  produced  in  the  quantity  and 
quality  of  the  chemical  activities  of  the  body  have  already 
been  fully  described  in  previous  chapters.  Muscular  work 
increases  the  oxidation  of  fats  and  carbohydrates,  and  when 
these  are  not  present  in  sufficient  quantities  to  yield  energy 
for  the  work,  it  also  increases  the  disintegration  of  proteid. 
Exposure  to  cold  seems  to  have  the  same  effect.  As  to 
the  influence  of  the  third  factor  mentioned  above,  —  the 
act  of  feeding  itself,  —  it  will  be  recalled  that  the  quantity 
of  proteid  disintegrated  depends  on  the  quantity  fed,  and 
that  the  quantity  of  fat  stored  depends  chiefly  on  the 
quantity  of  fat  and  carbohydrate  eaten. 

25.  Feeding  for  Muscular  Activity.  —  The  hard-working 
man  must  be  well  fed  if  he  is  to  do  his  best  work.  Not 
only  is  his  appetite  greater,  but  he  cannot  continue  hard . 
work  successfully  for  any  length  of  time  on  a  meager  diet. 
No  further  argument  is  needed  to  show  that  the  diet  in 
muscular  work  should  be  relatively  abundant.  But  should 
its  character  also  be  changed  ?  From  the  fact  that  when  the 
diet  remains  unchanged  muscular  work  increases  the  oxi- 
dation of  fats  and  carbohydrates,  while  it  affects  that  of 
proteid  only  as  the  other  nutrients  fail,  some  physiologists 
have  concluded  that  the  increased  food  taken  should  be 
non-proteid  rather  than  proteid.  It  is,  however,  actually 
found  that  when  those  who  are  engaged  in  hard  work 
are  free  to  choose  their  diet  according  to  the  dictates  of 
appetite,  they  use  more  proteids  as  well  as  more  carbo- 
hydrates and  fats.  Similarly  it  has  been  found  that  when 
a  race  horse  is  being  fed  for  the  track  he  does  better  work 


236  THE  HUMAN  MECHANISM 

when  fed  on  those  grains  which  (like  oats)  contain  rather 
large  proportions  of  proteid  than  on  those  which  (like 
corn)  contain  small  proportions  of  proteid.  This  prob- 
ably means  that  any  animal,  man  or  horse,  doing  hard 
work  is  better  off  for  having  more  living  flesh  with  which 
to  do  that  work  easily,  and  this  flesh  can  be  gained  (see 
p.  222)  only  by  eating  more  proteid.  Moreover,  when  once 
the  flesh  weight  has  been  increased  to  the  desired  amount 
it  can  be  maintained  only  by  continuing  with  the  same 
quantity  of  proteid  food  (p.  223).  While,  therefore,  it  is 
probable  that  the  greater  part  of  the  increase  of  the  diet 
for  steady  and  continued  muscular  work  should  fall  on  the 
fats  and  carbohydrates,  it  is  reasonable  to  eat  more  proteid 
and  thus  maintain  a  greater  flesh  weight.  When,  on  the 
other  hand,  the  "  greater  work  "  consists  of  a  day  or  so  of 
increased  effort  by  one  who  is  otherwise  engaged  only  in 
moderate  muscular  exertion,  there  is  not  the  time  for  an 
increase  of  flesh,  and  it  is  not  clear  that  any  nutritional 
gain  would  accrue  from  eating  more  proteid  after  the  day 
of  work  ;  at  the  same  time  there  is  no  reason  to  think  that 
harm  would  result  from  a  moderate  increase  of  this  nutrient. 

In  this  connection  it  is  easy  to  see  why  muscular  work 
should  antagonize  the  accumulation  of  fat  in  the  body, 
since  it  involves  the  consumption  of  those  foods  (carbohy- 
drates and  fats)  which,  when  present  in  excess  of  require- 
ments, furnish  the  material  for  the  manufacture  and  storage 
of  fat.  Hence  the  most  reasonable  treatment  of  obesity  is 
to  limit  the  use  of  non-proteid  foods  and  to  indulge  in  hard 
muscular  work. 

26.  Feeding  for  Cold  and  for  Warm  Weather.  —  The 
appetite  is  normally  better  in  cool  weather  than  in  warm, 
and  this  corresponds  to  the  greater  need  for  food,  one 
function  of  which  is  to  supply  by  its  oxidation  the  heat 
necessary  to  maintain  the  temperature  of  the  body.  More- 
over, we  tend  to  do  less  muscular  work  in  warm  weather 


NUTEITION  237 

than  in  cold,  and  this  is  an  important  factor  in  the  prob- 
lem. The  needed  increase  of  heat'  on  cold  days  may  be 
obtained  from  the  increased  use  of  fats  and  carbohydrates, 
although  there  would  seem  to  be  no  good  reason  why  pro- 
teids  may  not  also  be  increased.  We  have  seen  that  eat- 
ing more  proteid  readily  leads  to  the  oxidation  of  more 
proteid,  and  so  to  the  production  of  more  heat.  Hence,  in 
warm  weather,  when  the  body  finds  difficulty  in  securing 
the  needful  output  of  heat,  it  is  unwise  to  indulge  too 
freely  in  proteid  foods,  since  their  very  presence  within 
the  system  necessarily  increases  the  amount  of  heat  to  be 
got  rid  of.  There  is  a  widespread  popular  impression  that 
it  is  not  wise  to  eat  as  much  meat  in  summer  as  in  winter, 
and  the  impression  is  probably  correct,  although  it  applies 
also  to  vegetable  foods,  like  beans  and  peas,  which  con- 
tain relatively  large  quantities  of  proteid.  For  a  discus- 
sion of  the  relative  value  of  fats  and  carbohydrates  in  cold 
and  warm  climates,  see  p.  216. 

27.  Feeding  for  Mental  Work  and  Sedentary  Occupations. 
—  Mental  work  requires  special  attention  to  diet  chiefly 
for  two  reasons  :  in  the  first  place  it  involves  for  the 
greater  part  a  sedentary  life,  and  in  the  second  place  the 
processes  of  digestion  are  liable  to  interfere  more  or  less 
with  the  most  effective  working  of  the  brain.  Thus  far  no 
effect  of  mental  work  upon  the  character  of  the  chemical 
changes  within  the  body  has  been  conclusively  proved 
beyond  those  effects  which  are  obviously  the  result  of  the 
sedentary  life  which  accompanies  it.  Hence  feeding  for 
mental  work  is  largely  a  question  of  feeding  for  the  seden- 
tary life.  We  do  not  then  need  so  much  food,  and  espe- 
cially do  not  need  so  much  non-proteid  food.  To  eat  as 
much  carbohydrate  and  fat  as  when  we  are  leading  an 
active  life  would  be  to  have  these  nutrients  present  in 
excess,  and  thus  to  lead  either  to  the  storage  of  fat  or  else 
to  overtaxing  the  digestive  organs  which,  in  the  absence 


238  THE  HUMAN  MECHANISM 

of  reasonable  muscular  activity,  may  not  be  able  to  do 
their  best  work. 

This  being  the  case,  it  is  evidently  advisable  to  use 
rather  easily  digestible  food,  especial  attention  being  paid 
to  its  proper  preparation  in  cooking.  Pork  and  beans,  for 
example,  may  do  well  enough  for  men  working  on  a  farm 
in  cold  weather,  but  they  are  not  the  best  foods  for  a 
clerk  who  is  confined  to  his  office  during  the  greater  part 
of  the  day,  or  for  the  college  student.  Moreover,  special 
attention  should  be  paid  to  the  eating  of  food.  "  Quick 
lunches,"  inadvisable  at  all  times,  should  then  be  avoided 
altogether,  and  the  food  should  be  well  chewed.  These 
matters  will  be  fully  discussed  in  the  chapter  on  Alimen- 
tation, or  Right  Feeding  (Part  II). 

There  is  also  every  reason  to  believe  that  mental  activity, 
especially  when  it  involves  certain  feelings  or  moods,  may 
exert  no  inconsiderable  effect  on  the  nervous  processes  of 
digestion.  We  have  seen  that  the  secretion  of  gastric  juice 
depends  to  a  large  extent  upon  the  enjoyment  we  obtain 
during  the  act  of  eating;  and  the  man  or  woman  whose 
absorption  in  mental  work,  or  in  the  cares  and  annoyances 
of  life,  is  so  complete  that  the  food  is  eaten  with  little 
more  attention  to  its  taste  and  flavor  than  if  it  were  so 
much  paper,  is  certainly  laying  the  foundation  of  indiges- 
tion, and  so  of  bad  nutrition. 

The  above  considerations  point  clearly  to  the  need  of 
special  attention  to  diet  during  mental  occupations,  using 
that  term  in  its  broader  sense  to  include  all  occupations 
which  involve  mental  rather  than  muscular  work.  It  must 
not  be  supposed,  however,  that  we  can  correct  the  effects 
of  a  sedentary  life  by  mere  attention  to  diet.  As  we 
shall  show  in  Part  II,  the  human  mechanism  is  adapted 
to  a  life  of  muscular  activity,  and  on  the  whole  cannot 
remain  healthy  for  a  long  time  without  reasonable  use 
of  this  factor  of  health;  it  is  also  adapted  to  the  use  of 


NUTRITION  239 

a  reasonable  amount  of  rest,  and  cannot  thrive  when 
subjected  to  undue  nervous  strain.  There  are  times  in 
every  life  when  we  cannot  get  all  the  muscular  activity 
and  rest  we  need,  and  when  we  must  necessarily  be  sub- 
jected to  severe  nervous  strain.  When  such  is  the  case 
the  special  measures  we  have  outlined  should  be  taken 
with  regard  to  diet ;  but  we  should  not  be  deceived  into 
thinking  that  attention  to  diet  can  in  the  long  run  replace 
other  essential  factors  of  healthy  living.  In  other  words, 
normal,  healthy  nutrition  depends  not  alone  on  the  amount 
of  proteids,  fats,  and  carbohydrates  we  eat,  not  alone  on 
their  preparation,  not  alone  on  the  manner  in  which  we 
take  them,  nor  alone  on  all  these  things  taken  together  ; 
it  depends  no  less  on  the  hygienic  conduct  of  life  in  all 
respects.  The  best  nutrition  is  in  the  end  the  same  thing 
as  the  best  health,  and  this  is  assured  only  to  those  who 
"  keep  the  whole  law." 

28.  General  Conclusions  as  to  the  Choice  of  Nutrients.  - 
We  may  sum  up  as  follows.  The  amount  of  fats  and  carbo- 
hydrates needed  depends  primarily  on  the  amount  of  mus- 
cular work  done,  on  the  amount  of  heat  needed  to  maintain 
the  constant  temperature  of  the  body,  and  (in  cold  climates) 
on  the  quantity  of  subcutaneous  fat  desirable  to  assist  in 
preventing  undue  loss  of  heat  from  the  internal  organs. 
The  amount  of  proteid  needed,  on  the  other  hand,  is  not 
so  easily  determined.  Increased  proteid  feeding  increases 
proteid  destruction  within  the  body  and  leads  to  a  greater 
flesh  weight.  The  human  machine  will  continue  to  live 
and  do  its  work  on  very  different  quantities  of  proteid  in 
the  food ;  that  is  to  say,  when  it  disintegrates  very  different 
quantities  of  proteid  and  when  it  possesses  very  different 
flesh  weights.  Just  what  quantity  is  best  is  a  question  to 
which  the  facts  at  present  at  our  disposal  do  not  justify  a 
final  answer.  But  we  are  within  safe  limits  when  we  say 
that  it  is  well  to  avoid  both  extremes ;  too  much  proteid 


240  THE  HUMAN  MECHANISM 

probably  throws  undue  work  on  the  organs  of  excretion ; 
too  little  leads  to  an  undesirable  reduction  in  the  living 
flesh  or  working  equipment  of  the  body. 

In  the  foregoing  pages  we  have  refrained  from  giv- 
ing definite  figures  as  to  the  amount  of  each  nutrient 
needed  under  different  conditions  of  life,  partly  because 
such  figures  mean  but  little  to  those  who  can  go  no  more 
extensively  into  the  subject  than  the  majority  of  those  who 
read  this  book,  but  chiefly  because  any  figures  we  may 
give  are  apt  to  be  misleading.  So  much  depends  on  in- 
dividual peculiarities  of  height  and  general  build  of  the 
body,  and  so  much  on  the  occupations  and  other  condi- 
tions of  life,  that  to  deal  with  the  subject  effectively  would 
take  us  far  beyond  the  limits  of  the  present  work.  In 
actual  practice  we  cannot  depend  upon  the  chemical  bal- 
ance and  the  use  of  dietary  tables  to  secure  the  correct 
proportion  of  nutrients  in  our  food;  but  knowing  the 
general  composition  of  our  foods,  we  can  by  judicious 
selection  vary  the  amount  of  nutrients,  and  thus,  with  the 
help  of  appetite,  adapt  our  diet  more  or  less  successfully 
to  the  varying  conditions  of  life.  To  change  the  proteid 
we  have  only  to  change  the  quantity  of  those  foods  like 
meat,  eggs,  and  beans,  which  contain  relatively  large  quan- 
tities of  proteid  ;  and  we  similarly  vary  the  quantity  of 
non-proteid  by  changes  in  the  consumption  of  foods  like 
bread,  potatoes,  rice,  butter,  etc.,  which  contain  relatively 
large  amounts  of  carbohydrates  and  fats. 

29.  The  Question  of  Vegetarianism.  —  Closely  connected 
with  the  matters  discussed  in  the  present  chapter  is  the 
question  whether  the  human  machine  thrives  best  on  a 
mixed  diet  or  on  vegetable  food  alone.  No  one  would 
seriously  suggest  that  man  should  live  on  animal  food 
alone  ;  it  is  true  the  Eskimos  do  this,  but  with  them  it 
is  a  case  of  necessity,  and  no  one  would  recommend  to 
people  at  large  the  adoption  of  their  diet.  In  animal  foods 


NUTRITION  241 

the  non-proteid  nutrients  are  represented  chiefly  by  fat, 
and  we  have  every  reason  to  believe  that  an  undue  amount 
of  fat  in  the  diet  severely  taxes  the  digestive  powers ;  ani- 
mal foods  also  contain  too  little  indigestible  residue  to 
give  the  proper  stimulus  to  the  movements  of  the  alimen- 
tary canal. 

Vegetable  foods,  on  the  other  hand,  generally  contain 
enough  indigestible  cellulose  to  meet  the  latter  require- 
ment, and  most  of  them  also  contain  all  three  nutrients. 
With  few  exceptions,  however,  the  carbohydrates  are 
present  in  marked  excess.  In  order  to  obtain  from  such 
foods  the  proper  quantity  of  proteid,  it  is  generally  neces- 
sary to  eat  too  much  of  carbohydrate.  Some  vegetable 
foods,  notably  peas  and  beans,  are  not  open  to  this  objec- 
tion since,  although  they  have  a  very  high  percentage  of 
carbohydrate,  they  have  also  a  relatively  higher  percentage 
of  proteid;  and  it  is  possible  by  judicious  selection  and 
mixture  of  foods  to  obtain  a  nutritious  diet  from  vegetable 
foods  alone. 

This  is  much  easier  to  do  if  milk  and  eggs  are  added  to 
the  diet,  and  many  people  who  call  themselves  vegetarians 
do,  as  a  matter  of  fact,  make  extensive  use  of  these  animal 
foods.  With  them  vegetarianism  means  simply  the  use 
of  a  diet  from  which  meat  is  excluded.  Such  persons  are 
not  really  inconsistent,  as  is  often  charged,  for  the  question 
of  vegetarianism  is  practically  the  question  whether  man 
should  or  should  not  eat  meat. 

Against  the  use  of  meat  two  different  lines  of  argument 
are  used.  The  first  is  that  man  has  no  right  to  take  animal 
life  when  he  can  obtain  his  food  without  doing  so.  With 
this  argument  we  have  here  no  concern,  since  we  are  dealing 
solely  with  the  physiological  and  hygienic  aspects  of  the 
subject.  People  who  have  conscientious  scruples  against 
the  use  of  meat  may  fulfill  every  requirement  of  nutri- 
tion without  going  contrary  to  the  dictates  of  conscience. 


242  THE  HUMAN  MECHANISM 

The  second  argument  is  that  animal  food  is  not  a  healthy 
or  wholesome  food,  and  to  this  we  must  give  some  atten- 
tion. It  is  claimed  that  while  the  animal  proteids  and  fats 
may  be  unobjectionable,  meat  contains  other  substances 
which  exert  an  unfavorable  action  on  the  body.  Although 
it  is  true  that  meat  contains  small  quantities  (about  one 
per  cent)  of  organic  substances  other  than  proteids,  carbo- 
hydrates, and  fats,  it  has  not  been  proved  that  any  of  them 
are  harmful  to  a  healthy  man  or  woman.  Nor  is  it  an  answer 
to  this  argument  to  urge  that  in  gout  and  rheumatism  meat 
is  often  limited  or  even  prohibited  by  the  physician.  No 
one  —  not  even  a  vegetarian  —  would  suggest  that  starchy 
foods  should  be  given  up  because  persons  suffering  from 
diabetes  must  use  them  sparingly.  In  other  words,  it  has 
not  been  shown  that  the  eating  of  meat  is  the  cause  of 
rheumatism,  nor  even  that  it  is  one  of  the  causes  of  the 
disease ;  on  the  contrary,  it  is  more  in  accord  with  our 
knowledge  of  the  subject  to  regard  the  disease  as  originat- 
ing in  other  causes ;  and,  only  when  it  has  once  obtained 
a  foothold  in  the  body,  is  it  rendered  worse,  or  its  treat- 
ment more  difficult,  by  the  use  of  meat.  It  does  not  seem, 
therefore,  that  any  valid  hygienic  reason  has  yet  been  given 
for  excluding  all  meat  from  the  human  dietary. 

Nevertheless,  as  a  standing  protest  against  some  care- 
less habits  of  feeding,  vegetarianism  has  a  certain  value. 
Too  frequently  the  quantity  of  meat  eaten  during  a  meal 
is  out  of  all  proper  proportion  to  the  bread  and  vegetables. 
This  means  that  the  proteid  of  the  diet  is  in  excess,  that 
the  food  is  too  concentrated,  and  that  there  is  an  insuffi- 
cient quantity  of  indigestible  waste  to  exert  a  proper  laxa- 
tive effect  on  the  movements  of  the  intestine.  Take,  for 
example,  some  of  our  "  course  "  dinners,  —  oysters,  soup, 
fish,  roast,  game,  dessert  (perhaps  mince  pie),  crackers  and 
cheese,  coffee.  Here  nearly  every  course  consists  of  some 
form  of  proteid,  while  only  a  few  contain,  in  addition,  a 


NUTRITION  243 

small  quantity  of  vegetables.  A  very  common  method 
of  ordering  meals  at  a  restaurant  is  to  order  a  heavy  por- 
tion of  meat,  —  a  sirloin  steak  or  mutton  chops,  —  and  to 
make  the  meal  largely  from  this  proteid  food,  while  the 
vegetables  play  only  a  minor  r61e.  It  is  against  such  things 
as  these  that  vegetarianism  properly  protests  ;  but  there 
seems  to  be  no  good  reason  why  all  should  abstain  from 
meat  because  some  people  eat  it  to  excess.  In  general, 
poorly  cooked  vegetables  are  less  digestible  than  poorly 
cooked  meat.  Hence  to  attempt  to  live  on  a  purely  vege- 
table diet,  under  the  actual  conditions  of  modern  life,  is 
to  run  serious  risk  of  impairing  the  digestion.  We  should 
welcome  all  attempts  to  improve  the  preparation  of  vege- 
tables and  all  other  foods  by  better  cookery,  and  vegetarians 
have  done  much  to  help  forward  this  reform.  But  so  long 
as  the  actually  available  food  is  cooked  as  it  is,  it  is  wiser 
to  continue  the  mixed  diet  of  meat  and  vegetable  foods 
which  man  has  freely  chosen  in  the  past,  and  against  whose 
reasonable  use  no  valid  objections  have  yet  been  urged. 


CHAPTER  XIV 
SENSE  ORGANS  AND  SENSATIONS 

1.  The  Human  Mechanism  a  Conscious  Mechanism.  — 
Thus  far  we  have  repeatedly  compared  the  human  mechan- 
ism with  lifeless  mechanisms,  and  the  points  of  similarity 
are  most  interesting  and  instructive.    In  the  supply  of 
power,  the  elimination  of  wastes,  the  interdependence  and 
cooperation  of  parts,  the  adjustment  to  the  changing  con- 
ditions of  work,  and  in  many  other  respects  the  resem- 
blance holds  good.    But  in  one  respect  there  is  no  likeness 
whatever.    When  a  human  mechanism  is  not  in  good  work- 
ing order  or  is  tired,  it  may  be  aware  of  the  fact ;  when 
an  engine  is  damaged  in  any  way,  the  engine  does  not 
know  it.    Events  taking  place  in  the  living  animal  body 
arouse  in  it,  and  in  it  only,  conscious  sensations. 

Sensations  are  always  called  forth  by  the  condition  of 
some  organ  or  by  the  condition  of  the  body  as  a  whole. 
When  several  hours  have  passed  since  the  taking  of  food 
we  feel  hungry ;  or  of  drink,  we  feel  thirsty ;  when  any- 
thing touches  the  skin  a  sensation  of  touch  is  aroused;  if 
it  presses  very  hard,  that  part  of  the  skin  feels  painful ;  if 
the  tongue  is  acted  upon  by  sugar  or  salt,  we  get  a  sensa- 
tion of  taste  ;  if  light  enters  the  eye,  it  produces  conditions 
in  that  organ  which  arouse  in  us  sensations  of  color.  In 
all  these  cases  the  conscious  sensation  is  due  to  the  condition 
of  some  part  of  the  body. 

2.  The  Reference  of  Sensations.  —  Sometimes  we  refer 
the  sensation  to  the  part  of  the  body  which  is  first  affected, 
or  to  the  body  as  a  whole,  and  sometimes  we  refer  it  to 

244 


SENSE  ORGANS  245 

external  objects.  Thus,  if  in  driving  a  nail  the  hammer 
misses  the  nail  and  hits  a  finger,  we  refer  the  pain  to  the 
finger  and  not  to  the  hammer ;  and  we  similarly  refer  sen- 
sations of  hunger  and  thirst  to  the  body  and  not  to  exter- 
nal objects.  If,  on  the  other  hand,  the  skin  is  cooled  by  a 
piece  of  ice,  we  do  not  say  that  the  skin  is  cold,  but  that 
the  ice  is  cold ;  we  refer  the  sensation  to  the  external  object 
which  causes  it,  not  to  the  skin  in  which  it  actually  origi- 
nates. In  the  case  of  the  sense  of  sight,  this  reference  of 
the  sensation  to  the  external  object  which  sends  light  into 
the  eye  is  so  complete  that  unless  we  stop  and  reflect  upon 
it,  we  do  not  realize  that  it  is  the  condition  of  the  eye  of 
which  we  are  conscious,  rather  than  the  condition  of  the 
external  object  at  which  we  are  looking. 

3.  Sense  Organs.  —  A  few  sensations,   like   pain,    are 
aroused  by  the  condition  of  most,  if  not  all,  parts  of  the 
body;  there  is  no  one  organ  set  apart  to  produce  them. 
Some,  like  hunger,  although  at  times  more  or  less  general 
in  origin,  are  commonly  aroused  by  the  condition  of  some 
one   organ 1   which    ordinarily  performs   other  functions. 
Other  sensations  arise  in  organs  set  apart  for  the  purpose 
and  constructed  to  react  to  only  one  kind  of  stimulus  (spe- 
cial sense  organs,  or  organs  of  special  sensation).    To  this 
latter  class  belong  the  eye,  the  ear,  the  olfactory  mucous 
membrane  of  the  nose,  the  touch  organs  in  the  skin,  etc. 
We  therefore  speak  of  general  sensations  and  special  senses, 
although  no  sharp  line  of  division  can  be  drawn  between 
the  two. 

4.  The  Brain  the  Seat  of  Sensation.  —  In  all  cases  how- 
ever the  sensation,  although  originating  elsewhere,  is  de- 
veloped in  the  brain  and  not  in  the  sense  organ.    If  the 
optic  nerve  be  cut,  blindness  ensues,  although  light  falling 
on  the  retina  produces  the  same  effect  in  the  eye  itself  as 
when  the  nerve  is  intact ;  it  even  starts  nervous  impulses 

1  In  the  case  of  hunger,  the  stomach. 


246  THE  HUMAN  MECHANISM 

toward  the  brain ;  but,  since  these  impulses  go  no  farther 
than  the  cut,  they  excite  no  sensation  of  light.  And  the 
same  thing  is  true  of  other  sensations.  Conversely,  after  the 
amputation  of  a  limb  it  often  happens  that  sensations  are 
felt,  as  if  they  came  from  the  lost  member.  In  this  case 
the  stump  of  the  cut  nerve  is  stimulated  in  some  way,  and 
the  impulses  thus  sent  to  the  brain  excite  the  same  sensa- 
tions as  if  they  came  from  the  usual  endings  of  the  nerve. 
When  one  hits  his  "funny"  or  "crazy  bone"  (i.e.  directly 
stimulates  the  ulnar  nerve)  the  sensations  developed  in  the 
brain  may  be  referred  to  the  fingers  in  which  the  nerve 
originates. 

In  the  development  of  every  sensation,  therefore,  we 
have  to  distinguish  between  (a)  what  takes  place  in  the 
sense  organ  or  end  organ,  (b)  the  passage  of  a  nervous 
impulse  from  this  organ  to  the  central  nervous  system, 
and  (c)  the  events  which  the  arrival  of  the  nervous  impulse 
excites  in  the  brain.  It  is  only  the  last  (c)  that,  strictly 
speaking,  we  can  call  sensation.  The  sense  organs  and  their 
afferent  fibers  are  merely  tributary  mechanisms  which  serve 
to  excite  the  sensations  in  the  brain.  We  are  not  aware 
that  it  is  the  brain  which  is  thus  active,  for  we  refer  the 
sensation  either  to  the  organ  or  to  some  external  object. 

5.  The  Sense  of  Sight.  The  Eye.  —  Sight  is  one  of  the 
most  highly  specialized  of  the  senses.    The  eye  is  the  only 
organ  in  which  originate  sensations  of  light  or  color,  and 
it  is  a  wonderfully  constructed  apparatus,  the  function 
of  which  is  to  stimulate  the  optic  nerve  by  rays  of  light. 
It  is  essentially  a  living  camera  in  which,  by  means  of  a 
lens,  an  image  of  things  around  us  is  formed  upon  the 
retina ;  just  as  in  the  photographer's  camera  the  lens  forms 
an  image  on  the  ground  glass  or  on  the  sensitive  plate 
or  film. 

6.  Structure  of  the  Eye. — The  eyeball  consists  of  three 
concentric    coats  surrounding   and  inclosing    transparent 


SENSE  OEGANS  247 

substances  through  which  rays  of  light  pass  to  the  retina. 
The  outer  or  sclerotic  coat  (the  white  of  the  eye)  is  com- 
posed of  very  tough,  dense  connective  tissue,  and  forms 
the  protecting  covering  of  the  eye.  Over  a  small  area  in 
front  this  coat  is  transparent,  and  this  part  of  it  is  known 
as  the  cornea.  Inside  the  sclerotic  is  the  middle  coat  or 
choroid,  richly  supplied  with  blood  vessels  and  contain- 
ing in  its  connective  tissue  large  quantities  of  black  pig- 
ment, which  prevents  the  passage  of  light  into  the  eyeball 
except  through  the  cornea.  The  choroid  lines  the  scle- 
rotic everywhere  except  in  front,  where  in  the  region  of 
the  cornea  it  leaves  the  sclerotic  and  projects  toward  the 
long  axis  of  the  eye  as  a  kind  of  curtain,  the  iris,  —  that 
part  of  the  eye  which  is  black  or  gray  or  blue.  The  pupil 
is  the  dark  round  opening,  or  hole,  in  the  iris.  Immedi- 
ately inside  the  choroid  is  the  third  and  innermost  coat, 
the  retina.  This  is  a  thin  membrane,  not  more  than  one 
eightieth  of  an  inch  in  thickness,  and  lining  the  chamber 
of  the  vitreous  humor  as  far  forward  as  the  ciliary  region 
(Fig.  86).  The  retina  is  the  part  of  the  eye  sensitive  to 
the  stimulation  of  light.  Here  also  begin  the  fibers  of  the 
optic  nerve,  which  passes  through  and  perforates  the  choroid 
and  sclerotic  coats  behind  on  its  way  from  the  retina  to 
the  brain.  These  and  other  parts  of  the  eye  may  be  easily 
seen  by  dissecting  the  eye  of  an  ox  or  sheep. 

7.  The  Lens  and  the  Muscle  of  Accommodation.  —  Imme- 
diately behind  the  pupil  is  the  lens,  a  biconvex,  transparent, 
compressible,  and  elastic  body  fastened  by  a  circular  liga- 
mentous  sheet  to  the  choroid  coat  immediately  above  and 
behind  the  iris.  The  lens  arid  its  suspensory  ligamentous 
sheet  thus  divide  the  eye  into  two  distinct  chambers :  the 
one,  in  front  of  the  lens  and  behind  the  cornea,  filled  with 
a  watery  fluid,  the  aqueous  humor;  the  other,  behind  the 
lens  and  surrounded  by  the  retina,  filled  with  a  jellylike, 
transparent  substance,  the  vitreous  humor  (Figs.  86,  89). 


248 


THE  HUMAN  MECHANISM 


The  elastic  choroid  coat  is  not  long  enough  to  reach 
around  and  inclose  the  vitreous  humor  without  stretching, 
and  hence  it  constantly  exerts  a  steady,  elastic  pull  or  ten- 
sion on  the  ligament  of  the  lens.  This  tension  flattens  the 

compressible  lens  (i.e.  makes 
it  less  convex),  and  the  lens 
is  always  in  this  flattened 
condition  in  the  resting  eye  ; 
for  example,  when  one  is 
asleep.  The  same  condition 
should  obtain,  as  we  shall 
learn,  whenever  we  are  look- 
ing at  distant  objects. 

The  pull  of  the  tense  cho- 
roid on  the  lens  is,  however, 
overcome  at  times  by  the  ac- 
tion of  the  sheetlike  ciliary 
muscle.  The  fibers  of  this 
peculiar  muscle  originate  in 
the  sclerotic  coat  around  and 


Suspensory 
Ligament 


spective  view  into  the  hemisphere 
of  the  eyeball,  shown  in  Fig.  147 


FIG.  86.  Vertical  section  through  the 
ciliary  region  of  the  eye 

Showing  the  structures  concerned  in 
accommodation  (see  Section  7) .  This 
should  be  compared  with  the  per-  Just  OUtSlde  the  COmea,  and 

diverge  radially  outward 
and  backward  to  end  in  the 
choroid  beyond  the  attachment  of  the  suspensory  ligament 
of  the  lens.  Figure  87  shows  how  the  contraction  of  this 
muscle,  fixed  as  it  is  near  the  cornea,  must  draw  the  cho- 
roid forward  and  so  ease  the  pull  of  the  latter  on  the  liga- 
ment of  the  lens.  When  this  happens,  the  lens,  owing  to 
its  own  elasticity,  assumes  its  independent  (more  convex) 
shape. 

The  curvature  of  the  lens  is  thus  variable,  and  is  deter- 
mined by  the  action  of  this  muscle  of  accommodation. 
When  the  ciliary  muscle  is  at  rest,  the  lens  is  kept  flattened 
by  the  pull  of  the  choroid  on  the  ligament;  when  the 
muscle  contracts,  this  pull  is  eased  off  (or  slacked)  and 


SENSE  ORGANS 


249 


the  lens  becomes  more  convex.  The  entire  operation  is 
known  as  accommodation,  and  we  may  now  inquire  what 
part  accommodation  plays  in  vision. 

8.  The  Formation  of  an  Image  by  a  Lens.  —  The  eye  is 
a  camera,  in  that  it  forms  on  the  retina  an  image  of  objects 
in  front  of  the  cornea;  and  it  is 
the  first  essential  of  clear  vision, 
just  as  it  is  the  first  essential  of 
photography,  that  this  image  be 
sharp,  or  at  least  distinct.  A  sim- 
ple experiment  will  show  that  clear 
vision  of  near  and  of  distant  objects 
cannot  be  had  by  the  eye  at  the 
same  time.  Hold  up  a  pencil  or  a 
pen  about  ten  inches  from  the  eye 
and  look  first  at  it  and  then  at 
some  object  far  away.  Both  can  be 
seen,  but  only  one  at  a  time  clearly ; 

and  often  an  effort  is  required  to  FIG.  87.  Diagram  of  the 
shift  from  the  far  to  the  near  object. 

The  change  which  occurs  in  the 
eye  in  the  act  of  accommodation  is 
illustrated  in  the  following  experi- 
ment. A  wooden  or  pasteboard  box 
(approximately  8  by  5  by  4  inches)  is  fitted  with  a  piece 
of  ground  glass  on  one  side  and  provided  with  a  convex 
lens  on  the  opposite  side.  This  is  a  rude  camera,  and 
some  object  is  now  placed  at  such  a  distance  that  the  lens 
forms  an  image  of  it  on  the  ground  glass,  which  is  now  in 
focus  for  the  object.  If,  later,  the  object  be  moved  nearer 
to  the  lens,  the  focus  is  changed ;  the  image  on  the  glass 
becomes  blurred,  and  in  order  to  make  it  distinct  it  will 
be  found  necessary  to  use  a  more  convex  lens. 

Essentially  the  same  change  occurs  in  the  eye  in  accommo- 
dating for  near  objects  :  the  lens  must  be  made  more  convex  ; 


mechanism  of  accommo- 
dation 

The  ciliary  muscle  is  repre- 
sented as  three  fibers  pass- 
ing obliquely  from  the 
sclerotic  to  the  choroid 


250  THE  HUMAN  MECHANISM 

and  this,  it  will  be  remembered,  involves  work  on  the 
part  of  the  muscle  of  accommodation  (see  p.  248).  We 
can  thus  understand  why,  in  general,  it  is  too  much  of 
"  near  work,"  and  especially  near  work  necessitating  very 
distinct  vision,  that  tires  the  eye.  The  ideal  condition  of 
the  eye,  regarded  merely  as  a  camera,  is  that  in  which  dis- 
tant objects  are  focused  on  the  retina  when  the  muscle  of 
accommodation  is  completely  relaxed  and  the  lens  is  thus 
flattened  to  its  utmost  by  the  elastic  pull  of  the  choroid  coat 


V 


FIG.  88.  Action  of  a  convex  lens  in  bringing  to  a  focus  the  rays  of  light 

diverging  from  a  single  point 
The  rays  from  A  are  focused  at  a ;  those  from  B,  at  6 

(p.  248) ;  for  in  this  case  the  eye  is  rested  by  looking  at  dis- 
tant objects,  and  works  only  when  looking  at  near  objects. 
Such  an  eye  is  known  as  an  emmetropic  eye  (Fig.  90,  IS). 

Unfortunately,  not  all  eyes  meet  this  requirement.  The 
eyeball  may  be  either  too  short  or  too  long ;  so  that,  with 
the  muscle  of  accommodation  relaxed,  the  position  of  the 
perfect'  focus  for  distant  objects  is  either  before  or  behind 
the  retina ;  the  eye  no  longer  sees  distant  objects  distinctly 
when  it  is  at  rest,  because  then  the  retinal  image  is  blurred. 
To  understand  more  fully  the  undesirable  consequences 
of  this  condition,  we  must  learn  how  convex  lenses  produce 
images  of  objects. 

9.  The  Action  of  a  Convex  Lens  on  Rays  of  Light.  —  The 
rays  of  light  diverging  from  a  single  point  and  entering  a 


SENSE  OKGANS  251 

convex  lens  are  bent  so  that  all  come  together  again  in  a 
point  behind  the  lens,  or,  as  it  is  said,  are  brought  to 
a,  focus.  This  is  shown  in  Fig.  88,  as  is  also  the  fact  that 
rays  of  light  diverging  from  more  distant  points  come  to  a 
focus  behind  the  lens  sooner  than  those  diverging  from 
nearer  points. 

Now  a  lens  forms  an  image  of  an  object  because  all  the 
rays  of  light  from  each  point  of  the  object  are  focused  in 


ch 


ch 


FIG.  89.  Diagram  showing  the  formation  of  an  -image  on  the  retina 

1,  2,  the  object;  1',  2',  the  image  of  the  same;  c,  cornea;  i,  iris;  I,  lens; 
v,  vitreous  humor;  w,  sclerotic;  ch,  choroid;  on,  optic  nerve 

corresponding  points  behind  the  lens.  This  is  shown  in 
Fig.  89,  where  all  the  rays  diverging  from  1  are  focused  at 
1',  all  those  from  2  at  2',  and  those  from  intermediate 
points  of  the  object  at  intermediate  points  of  the  image. 

If  the  rays  from  each  point  meet  in  front  of  the  retina 
and  then  diverge  before  reaching  the  retina,  the  retinal 
image  is  blurred ;  and  the  image  is  also  blurred  if  the 
retina  is  so  near  the  lens  that  the  rays  from  each  point 
have  not  yet  come  to  a  focus.  The  more  convex  the  lens 
the  more  will  the  rays  of  light  be  bent ;  consequently  we  use 
the  muscle  of  accommodation  (which  makes  the  lens  more 
convex)  to  get  clear  images  of  near  objects  (see  Fig.  88). 


252  THE  HUMAN  MECHANISM 

10.  Myopia,  Hypermetropia,  and  Presbyopia.  —  In  the 

emmetropic  eye  (Fig.  90,  E)  the  distance  between  the 
retina  and  the  lens  is  such  that  light  from  distant  points 

comes  to  a  focus  on 

the  retina  without 

any  active  muscular 

accommodation;  to 

see  near  objects  the 
lens  is  made  more 
convex. 

When  the  retina 

is  so  far  away  from 

the  lens  that,  with 

the  muscle  of  ac- 
commodation com- 
pletely relaxed  and 

^^_  .      therefore    the    lens 

^-fC^         J  flattened  to  its  ut- 

most,  light  from  dis- 
tant points  comes  to 
FIG.  90.  Course  of  the  rays  of  light  from  a    a  focus  in  front  of 
distant  point  the  retina,  the  reti- 

Through  the  emmetropic  (E),  the  myopic  (M),  and  naj  imap'eis  blurred 
the  hy permetropic  (H)  eye,  the  muscle  of  accom-  ... 

modation  being  relaxed.     (The  rays  diverging  and  it  is  impossible 

from  a  distant  point  would  enter  the  eye  practi-  fnr   Q110V,    art    PVA   fn 

J.L/L      &  LlUll     till     t?  y  C5      \j\J 

cally  parallel)  ^ . 

see  distant  objects 

clearly.  To  correct  such  vision  it  would  be  necessary  to 
make  the  lens  still  less  convex,  and  this  the  eye  is  unable 
to  do.  (Why?)  Such  an  eye  is  known  as  myopic,  or  near- 
sighted, and  its  defect  must  be  corrected  by  the  use  of  con- 
cave glasses,  which  act  as  if  the  lens  were  made  flatter,  and 
so  throw  the  focus  farther  back  upon  the  retina.  A  myopic 
eye  generally  has  clear  sight  for  very  near  objects  because, 
as  stated  above,  the  nearer  the  object  the  farther  back  is 
the  image  formed. 


SENSE  OEGANS  253 

On  the  other  hand,  the  eyeball  may  be  too  short,  fore 
and  aft  (Fig.  90,  H),  so  that,  when  the  ciliary  muscle  is 
relaxed,  light  from  distant  points  has  not  yet  been  brought 
to  a  focus  when  it  reaches  the  retina  (Jiypermetropia). 
Such  an  eye  must  accommodate  not  only  for  near  but  also 
for  distant  objects,  and  its  muscle  of  accommodation  can 
never  rest  so  long  as  the  eye  is  being  used.  Moreover, 
to  see  near  objects  the  ciliary  muscle  must  work  much 
harder  than  in  the  normal  eye,  and  it  often  happens  that, 
even  with  its  utmost  effort,  the  rays  are  not  sufficiently 
bent  to  focus  them  on  the  retina ;  so  that  a  book,  for  ex- 
ample, must  be  held  at  arm's  length  to  be  read.  Persons 
having  such  eyes  form  one  class  of  those  said  to  be  "  far- 
sighted,"  and  their  trouble  can  be  corrected  by  the  use  of 
convex  glasses. 

As  old  age  approaches,  changes  occur  in  the  lens,  in 
consequence  of  which  it  no  longer  becomes  as  convex  as 
formerly  in  response  to  the  action  of  the  muscle  of  accom- 
modation (presbyopia,  from  Trpeafivs,  old).  Some,  though 
not  all,  results  of  this  condition  resemble  those  of  hyper- 
metropia  ;  but  the  two  differ  in  cause.  Hypermetropia  is 
due  to  shortness  of  eyeball;  presbyopia,  to  failure  of  accom- 
modation. 

11.  Astigmatism.  —  We  have  thus  far  been  dealing  with 
those  optical  imperfections  due  to  improper  distance  be- 
tween the  lens  an,d  the  retina.  Another  and  frequently 
more  serious  trouble,  known  as  astigmatism,  results  when 
the  curvature  of  the  cornea  (and  sometimes  of  the  lens)  is 
not  perfectly  regular  ;  i.e.  when  these  surfaces  are  not  seg- 
ments of  perfect  spheres,  but  resemble  in  curvature  the 
side  of  a  lemon.  In  this  case  the  rays  of  light  from  a  point 
are  not  brought  to  a  focus  again  in  a  point  behind  the 
lens;  and  remembering  the  importance  of  sharp  focusing  in 
securing  distinct  retinal  images,  the  student  will  see  that 
this  defect  must  seriously  interfere  with  clear  vision.  The 


254 


THE  HUMAN  MECHANISM 


optics  of  astigmatism  are  too  complicated  to  be  explained 
in  an  elementary  work,  but  the  defect  reveals  itself  gener- 
ally in  an  inability  to  see  with  equal  clearness  lines  run- 
ning in  different 
directions.  Thus 
some  of  the  lines 
in  Fig.  91  will  be 
sharply  denned  and 
black  while  one  is 
looking  with  one 
eye  at  the  white 
center,  and  others 
will  be  blurred  and 
lighter  in  color. 

Astigmatism    is 
of    special    impor- 
tance in  reading, 
because  the  lines  of 
FIG.  91.  A  test  for  astigmatism  printed  letters  run 

in  different  direc- 
tions. The  effort  to  see  clearly  the  printed  page  is  often 
severe,  and  results  in  headaches  and  other  general  dis- 
turbances of  health,  the  true  cause  of  which  is  often  un- 
suspected. The  trouble  may  usually  be  corrected  by  the 
use  of  so-called  "cylindrical"  glasses,  i.e.  glasses  which 
compensate  the  defects  of  curvature  in  lens  and  cornea. 

12.  Accommodation  and  "Near  Work."  —  The  above- 
described  defects  of  the  eye  as  an  optical  instrument  may 
usually  be  successfully  corrected  by  the  use  of  proper 
glasses,  which  should,  generally  speaking,  be  prescribed 
by  a  good  oculist  and  not  by  an  optician.  Glasses  may  be 
used  for  various  reasons,  —  as  a  matter  of  convenience,  as 
where  a  person  with  slight  myopia  wears  them  merely  to 
see  distant  objects  clearly;  or  of  necessity,  as  when  the 
myopia  is  more  pronounced  ;  or  they  may  serve  the  much 


SENSE  OKGANS  255 

more  important  purpose  of  relieving  the  muscle  of  accom- 
modation of  undue  work  in  reading  or  sewing,  and  thus 
of  avoiding  "  eye  strain."  A  hypermetropic  eye  should 
always  be  provided  with  glasses,  since  otherwise  its  muscle 
of  accommodation  cannot  be  rested  by  looking  at  distant  ob- 
jects. But  since  it  is  near  work  which  requires  the  greatest 
effort  of  accommodation,  it  is  in  reading,  writing,  drawing, 
sewing,  etc.,  that  the  eye  strain  is  apt  to  be  greatest.  As 
this  kind  of  work  is  constantly  increasing  in  modern  life,  the 
need  for  the  complete  correction  of  such  defects  becomes 
more  and  more  necessary.  Those  whose  occupations  require 
long-continued  use  of  the  eyes  should  see  to  it  that  these 
precious  organs  are  used  only  under  the  most  favorable 
conditions,  and  that  all  strain  is  as  far  as  possible  relieved. 

13.  The   Importance   of   Sufficient   Illumination,   etc.— 
When  we  try  to  read  in  a  poorly  lighted  room  we  natu- 
rally hold  the  printed  page  closer  to  the  eye  in  order  to  get 
from  its  bright  parts  the  greatest  possible  amount  of  light, 
and  also  because  the  images  thus  formed  are  larger.    But 
this  clearly  involves  more  severe  work  on  the  part  of  the 
muscle  of    accommodation   in   order  to  give  the  lens  its 
unusual  convexity.    Such  work  frequently  results  in  injury 
to  the  parts  concerned,  and  should  be  avoided  as  far  as 
possible.    Many  an  eye  has  been  injured,  especially  in  early 
life,  by  reading  too  late  in  the  waning  light  of  evening 
before  lighting  a  lamp. 

It  is  also  of  the  highest  importance  that  printed  matter 
be  held  steady,  as  the  effort  of  accommodation  is  otherwise 
rendered  more  difficult.  Consequently  it  is  in  general  a 
bad  thing  to  read,  and  especially  to  read  fine  or  poorly 
printed  matter,  on  any  but  the  steadiest  railroad  train. 
For  the  same  reason,  for  all  kinds  of  near  work  an  un- 
steady or  flickering  light  is  very  objectionable. 

14.  Visual  Sensations.  —  We  have  shown  (p.  245)  that 
the  sensation  of  sight  does  not  develop  in  the  eye,  but  in 


256  THE  HUMAN  MECHANISM 

the  brain,  as  the  result  of  nervous  impulses  sent  thither 
over  the  fibers  of  the  optic  nerve  from  the  retina.  Just 
how  the  light  falling  upon  the  retina  originates  these 
impulses  cannot  be  discussed  here ;  suffice  it  to  say  that 
the  character  of  the  impulse  differs  according  to  the  color 
of  the  light1  stimulating  the  retina;  the  lens  focuses  upon 
the  retina  a  flat,  colored  picture  of  the  objects  at  which  it 
is  looking,  just  as  a  photographic  camera  does,  or  as  the 
painter  represents  a  scene  on  canvas.  One  part  of  the  ret- 
ina is  thus  stimulated  by  light  of  one  color,  and  another 
part  by  light  of  another  color,  or  by  another  shade  of  the 
same  color ;  and  the  different  kinds  of  impulses  started  in 
the  fibers  of  the  optic  nerve  ultimately,  upon  their  aTrival 
in  the  brain,  excite  in  consciousness  what  we  know  as 
visual  sensations.  The  sensations  which  we  get  from  the 
retina  are  therefore  primarily  sensations  of  color. 

15.  Visual  Judgments. —  But  when  we  look  at  an  object 
we  get  more  than  mere  sensations  of  color.  The  world 
does  not  appear  to  us  as  a  flat  surface,  of  different  colors, 
like  the  painter's  canvas.  When  we  look  at  the  wall  of  a 
room  we  know  that  it  is  a  flat  surface,  and  when  we  look 
at  a  box  we  know  that  it  has  not  only  length  and  breadth 
but  also  thickness.  If  we  were  dependent  entirely  upon 
the  retinal  image  for  our  idea  of  the  box,  it  would  look 
as  flat  as  the  wall ;  that  it  does  not  appear  so  is  because 
we  receive  other  information  about  the  box  than  that 
which  comes  from  the  retina.  We  have  to  accommodate 
the  lens  differently  for  the  near  and  the  far  edges,  and  we 
have  learned  by  experience  that  this  necessity  indicates 
depth,  or  different  distances  of  different  parts  of  the  object. 
Again,  we  see  the  box  with  both  eyes,  and  the  images 
formed  on  the  two  retinas  are  not  exactly  the  same.  One 
eye  sees  more  of  one  side,  the  other  eye  more  of  another 

1  In  this  and  the  following  paragraphs  white,  black,  and  gray  are 
regarded  as  colors. 


SENSE  OKGANS  257 

side ;  and  while  we  are  not  conscious  of  this  fact,  we  have 
really  learned  by  experience  and  by  the  actual  handling  of 
objects  that  these  slightly  different  sensations  from  the 
two  eyes  are  produced  only  by  solid  objects.  Again,  when 
we  look  at  any  point  on  the  near  edge  of  'a  box  the  two 
eyes  are  converged  by  their  muscles  to  a  greater  extent 
than  when  we  look  at  a  point  on  the  far  edge,  and  we 
have  learned  that  these  different  pulls  of  muscles  and  posi- 
tions of  eyeballs  indicate  that  the  object  is  not  flat,  but  has 
depth.  The  importance  of  binocular  vision  in  the  estima- 
tion of  depth  or  distance  from  the  eye  is  most  strikingly 
illustrated  by  attempting,  with  one  eye  closed,  to  bring 
together  the  points  of  two  pencils  held  in  the  hands  and 
moved  from  side  to  side  at  arm's  length. 

Consequently  when  we  look  at  anything  we  get  a  num- 
ber of  sensations ;  from  the  retina,  those  of  color  and  the 
position  of  the  color  spots  with  reference  to  one  another; 
from  the  muscular  efforts  of  accommodation  and  of  con- 
vergence of  the  eyeballs,  those  which  reveal  the  property 
of  depth  in  what  we  see.  And  from  all  of  these,  fused 
together  and  interpreted  in  the  light  of  experience,  we 
construct  a  visual  judgment  of  the  nature  of  the  object. 

16.  Optical  Illusions.  —  That  our  vision  is  essentially  the 
result  of  unconscious  judgments  is  strikingly  shown  by 
the  fact  that  these  sometimes  deceive  us.  Thus  the  par- 
allel vertical  lines  in  Fig.  92,  when  crossed  by  the  oblique 
lines,  seem  to  be  inclined  toward  each  other.  The  retinal 
images  of  the  lines  are  parallel,  and  we  falsely  judge  them 
inclined,  this  error  of  judgment  arising  from  the  presence 
of  the  oblique  lines.  In  other  words,  our  final  idea  of  the 
lines  does  not  correspond  to  their  image  on  the  retina. 

Many  other  examples  might  be  given  showing  that  our 
visual  idea  of  the  world  around  us  is  not  a  simple  sensa- 
tion or  impression,  but  an  unconscious  inference,  judg- 
ment, or  conclusion  built  up  from  a  number  of  simple 


258 


THE  HUMAN  MECHANISM 


'5 


!3 


m 


* 

*    5 


/ 


! 


:M 


s 


sensations  taken  separately  or  blended  together,  and.  com- 
pounded with  results  of  lifelong  experience.  In  looking 
at  a  piece  of  fine  silk  or  cloth  we  seldom  stop  to  think 
that  its  tissue  may  be  resolved  into  many  simple  component 
threads ;  and  in  quite  the  same  way  we  fail  to  realize  that 

i even     our   quickly    formed 

judgments  of  the  size,  dis- 
tance, form,  or  color  of  ob- 
jects are  likewise  tissues 
woven  out  of  many  threads, 
most  of  which  we  have  been 
slowly  and  laboriously  spin- 
ning since  childhood's  days 
in  the  hidden  factory  of  in- 
dividual experience. 

17.  Sound  and  Hearing.— 
When  the  string  of  a  violin, 
piano,  or  harp  " sounds,"  one 
can  observe  that  it  is  in 
rapid  vibration  ;  and  the  same  thing  is  true  of  all  sounding 
bodies.  These  vibrations  are  imparted  to  the  air,  water,  or 
other  surrounding  medium,  and  through  this  medium  they 
are  transmitted  as  waves  of  sound.  It  is  these  waves  or 
vibrations  which,  on  entering  the  ear,  excite  the  sensa- 
tion of  sound.  The  more  rapid  the  vibrations  the  higher 
is  the  pitch  of  the  note,  and  the  greater  their  amplitude 
the  louder  the  sound. 

The  ear  is  an  organ  specially  adapted  to  receive  these 
vibrations  of  air  and  to  transform  them  into  nervous  im- 
pulses. It  is  subdivided  by  anatomists  into  the  'outer  ear, 
the  middle  ear,  and  the  inner  ear. 

18.  The  Outer  Ear.  -  -  The  outer  ear  consists  of  the 
expanded  pinna  (or  that  part  which  we  commonly  call 
"the  ear")  and  a  tube  along  which  the  vibrations  of 
sound  pass  inward  to  the  tympanic  membrane,  or  drum. 


FIG.  92 


SENSE  ORGANS 


259 


Glands  along  this  canal  secrete  wax  which  guards  the 
approach  to  the  drum.  It  is  a  bad  habit  to  pick  at  this 
wax,  and  especially  to  dig  into  the  ear  with  any  pointed 


FIG.  93.  Diagram  of  the  ear 

A,  the  auditory  canal,  leading  to  the  tympanic  membrane  B ;  (7,  cavity  of  the 
tympanum,  communicating  by  the  Eustachian  tube  with  the  pharynx  D ; 
E,  semicircular  canals ;  F,  cochlea ;  (?,  auditory  nerve 

instrument,  for  there  is  always  danger  of  perforating  the 
drum.  If  trouble  is  suspected,  a  physician  should  be 
consulted. 

19.  The  Middle  Ear.  The  Eustachian  Tube. —  The  tym- 
panic membrane  separates  the  outer  from  the  middle  ear, 
or  tympanum,  a  small  cavity  lying  in  the  temporal  bone  of 
the  skull  and  communicating  with  the  throat  or  pharynx  by 
means  of  the  Eustachian  tube.  The  air  which  it  contains 
is  consequently  under  the  same  pressure  as  that  of  the  at- 
mosphere without,  and  the  tympanic  membrane  is  not  nor- 
mally bulged  inward  or  outward  by  inequality  of  pressure 


260  THE  HUMAN  MECHANISM 

on  its  two  sides.  The  opening  of  the  Eustachian  tube  into 
the  pharynx  is,  however,  closed  except  when  one  swallows, 
and  hence  swallowing  often  relieves  the  drum  from  undue 
pressure  of  air  in  the  middle  ear. 

The  cavity  of  the  tympanum  also  communicates  with  a 
network  of  spaces,  or  sinuses,  in  the  temporal  bone.    Because 

of  these  connections  of  the 
middle  ear  with  the  throat,  on 
the  one  hand,  and  with  the 
temporal  sinuses  on  the  other, 
inflammatory  processes  in  the 
nose  and  throat  during  a  cold 
sometimes  extend  into  the 
Eustachian  tube,  the  tympa- 
num, and  even  into  the  tem- 

poral sinuses,  causing  serious 
FIG.  94.  The  bony   labyrinth,    its    .        i  -,  n 

actual  size  being  shown  in  the    tr°uble>    and    occasionally 

'smaller  figure  deafness. 

B,    C,  D,   the  semicircular  canals;          Passing  directly  across  the 

A  ,  the  oval  window,  by  means  of  tympanum,  from  the  drum  on 

which  the  vibrations  of  the  stirrup    .  J 

hone  are  transmitted  to  the  coch-    its   Outer  Side    to   the  Cochlea 


lea;   E,  F,  G,  the  whorls  of  the    Qn   itg   inner   gide     ig 
cochlea.    Compare  Fig.  95 

of  three  very  small  bones,  the 

ear  ossicles  (hammer,  anvil,  and  stirrup).  These  bones  are 
bound  together  and  attached  to  the  walls  of  the  tympanum 
by  ligaments,  and  are  so  arranged  that  when  sound  waves 
set  the  tympanic  membrane  in  vibration  this  motion  is 
transmitted  by  the  ossicles  to  a  portion  of  the  inner  ear 
known  as  the  cochlea. 

20.  The  Inner  Ear.  —  The  structures  of  the  inner  ear  lie 
in  the  temporal  bone,  on  the  side  of  the  tympanum  opposite 
the  drum.  They  consist  of  a  system  of  small  bony  spaces 
and  tubes,  the  bony  labyrinth,  within  which  lies  a  cor- 
responding membranous  labyrinth.  Forming  part  of  the 
lining  of  the  membranous  labyrinth  are  very  sensitive 


SENSE  OKGANS  261 

cells,  and  between  these  cells  are  the  endings  of  the  nerve 
fibers  which  connect  the  ear  with  the  brain.  The  cells  of 
the  inner  ear  are  sensitive  to  the  vibrations  which  have 
been  transmitted  vestibule  with  Openings 

across  the  tympa-  of  Semicircular  Canals 

num   by  the    os-  v(\\V/x'' 

Sides,  just  as  the  /*     ~I^X\        1  ^1    ScalaVestibuli 

retina  is  sensitive    jv/aSeusXC^T/  ^k      \ 

to  light;    and  as   sta^^^|^^W\ 

the  retina  is  the  ^^        ^^oO^yJ / Cochlea 

Origin  of  the  fibers    Jlfemfcranafym^anlV^          j^^^^^X 
Of  the  Optic  nerve,  "^^j^^^V  Sc^Tympani 

so   uiiG   iniiGr   cdi  ^^ 

is    the    origin    of  -FlG<  95>  Diagrammatic  representation  of  the  mem- 
"  £  ^  j*         branous  labyrinth  of  the  cochlea  in  relation  to 

the  audi-      the  structures  shown  in  Figs.  93  and  94 

tory  nerve.  j^  scaia  vestibuli  and  scala  tympani  are  the  two  por- 

21.  Taste  and        tions  of  the  bony  cochlea  which  inclose  the  mem- 

Smell. -The  end      branous  cochlea 

organs  of  taste  are  small  rounded  eminences,  or  papillce, 
on  the  dorsal  surface  of  the  tongue,  and  from  these  the 
fibers  of  the  nerves  of  taste  pass  to  the  brain.  The  end 
organs  of  the  nerve  of  smell  are  situated  in  the  upper  por- 
tion of  the  nasal  cavity  and  consist  of  delicate  cells  very 
sensitive  to  the  presence  of  odors.  Sensations  of  taste  are 
frequently  confounded  with  those  of  smell.  An  onion,  for 
example,  has  little  or  no  taste,  as  can  be  shown  by  placing 
a  bit  on  the  tongue  when  one  is  holding  the  breath ;  none 
of  the  flavor  of  the  onion  is  perceived.  On  the  other 
hand,  sour,  sweet,  bitter,  and  salt  are  true  taste  sensations. 
The  unconscious  blending  of  tastes  with  odors  in  forming 
our  ideas  of  the  nature  of  objects  recalls  the  formation 
of  visual  judgments  by  the  combination  of  retinal  with 
other  sensations  (p.  256). 

22.  Cutaneous  Sensations.  —  The  skin  is  the  place  of 
origin    of   at   least   three    sensations,  —  touch,    cold,   and 


262 


THE  HUMAN  MECHANISM 


warmth.  These  sensations  are  distinct,  as  is  shown  by 
the  observation  that  on  certain  points  of  the  skin  some 
of  them  may  be  felt,  but  not  others.  This  fact  is  usually 
interpreted  to  mean  that  each  sensation  has  its  own  set 
of  end  organs  and  nerve  fibers.  Especially  striking  is  the 
fact  that  warmth  and  cold  are  never  felt  by  the  same  spot 
of  skin,  which  seems  to  prove  conclu- 
sively that  they  are  separate  sensations. 
The  afferent  nerves  of  cold  and 
warmth  not  only  carry  into  the  brain 
those  impulses  which  give  rise  to  the 
corresponding  sensations  but  also  serve 
as  one  important  means  of  stimulating 
the  reflexes  which  help  to  regulate  heat 
production  and  heat  output  (see  Chap- 
ter XII). 

FIG.  96    A  tactile  cor-          33     The  Senge     f  position.  _  The  ex_ 

puscle  in  one  of  the 

papillse  of  the  der-    pression  "  the  rive  senses     has  become 

mis;  an  end  organ    proverbial,   and   comes   from   the   time 
when    gight>  hearing?    taflte?   smelli   and 

touch  were  the  recognized  special  senses. 
To-day,  however,  we  must  add  to  these  not  only  warmth 
and  cold  but  still  others,  most  conspicuous  among  which 
is  the  sense  of  position.  When  the  eyes  are  closed  we  are 
aware  of  the  position  of  the  various  parts  of  the  body.  We 
know  whether  the  arm  is  bent  at  the  elbow  or  straight ; 
whether  the  head  is  looking  forward  or  is  turned  to  one 
side  or  the  other.  And  while  we  are  aware  of  these  things, 
partly  from  tactile  sensations,  there  is  conclusive  evidence 
that  afferent  Impulses  from  the  muscles,  tendons,  and 
joints  also  play  an  important  part  in  the  result. 

When  one  is  blindfolded  and  lies  flat  on  a  revolving 
table  which  can  be  turned  noiselessly  in  one  direction  or 
the  other,  the  subject  of  experiment  can  form  fairly  correct 
judgments  as  to  the  angle  and  direction  through  which 


of  the  sense  of 
touch 


SENSE  OKGANS  263 

the  table  is  turned.  Here  there  is  no  change  of  character 
either  in  the  tactile  impulses  or  in  those  from  the  muscles, 
tendons,  and  joints,  for  the  subject  of  experiment  lies  still 
and  is  only  passively  moved.  It  is  believed  that  in  this 
case  the  sensations  in  question  come  from  the  movements 
of  the  lymph  in  portions  of  the  inner  ear.  One  part  of 
this,  the  cochlea,  is  undoubtedly  concerned  with  the  per- 
ception of  sound ;  but  another  part,  the  three  semicircular 
canals,  are  now  believed  to  be  end  organs  of  this  sense  of 
position. 

The  impulses  which  make  us  aware  of  the  position  of 
parts  of  our  bodies  also  play  a  very  important  r61e  in 
reflexly  guiding  our  movements.  Upon  this  we  shall  dwell 
at  greater  length  in  subsequent  chapters  (see  especially 
Chapter  XV). 

24.  Sensations  of  Pain.  —  Most  organs  of  the  body  may 
also  give  rise  to  impulses  which,  on  their  arrival  in  the 
brain,  cause  sensations  of  pain.  It  is  still,  perhaps,  an 
open  question  whether  this  sensation,  like  sight,  smell,  and 
hearing,  is  aroused  by  its  own  mechanism  of  end  organs 
and  afferent  nerves,  or  whether  it  is  called  forth  by  the 
excessive  stimulation  of  the  nerves  of  the  other  senses; 
but  for  the  discussion  of  this  question  the  reader  must 
consult  more  advanced  works  on  physiology. 

Pain  is  a  useful  danger  signal,  since  it  effectively  calls 
attention  to  abnormal  conditions  and  incites  us  to  the 
adoption  of  active  remedial  measures.  Remedies,  how- 
ever, should  not  be  confined  to  the  abolition  of  unpleasant 
sensations,  but  should  be  directed  to  the  removal  of  their 
cause.  A  toothache  from  a  decaying  tooth  may  often  be 
stopped,  for  a  time  at  least,  by  the  use  of  chloroform  or 
other  anesthetic  drugs,  but  the  drug  only  stops  the  pain ; 
it  does  not  check  the  progress  of  decay  or  repair  the 
damage.  Again,  a  bronchial  cough  may  be  unpleasant, 
and  even  painful,  but  we  should  not  rest  content  with  the 


264  THE  HUMAN  MECHANISM 

use  of  some  drug  or  cough  medicine  which  merely  lessens 
the  irritability  of  the  inflamed  surface  of  the  air  passages, 
and  so,  perhaps,  stops  the  cough  without  curing  the  disease. 

Pain  is  a  warning  that  some  abnormal  condition  needs 
attention.  Sometimes  that  attention  may  be  supplied  by  the 
sufferer  himself,  or  by  his  friends,  but  often  skilled  medical 
advice  is  needed.  Too  frequently,  for  the  sake  of  economy 
or  from  feelings  of  modesty,  or  even  because  of  an  unwilling- 
ness to  acknowledge  illness  either  to  the  world  or  to  one- 
self, the  mistake  is  made  of  postponing  the  visit  to  the 
physician,  the  patient  meanwhile  bearing  discomfort  and 
perhaps  actual  suffering  in  the  hope  that  he  will  soon  be 
better  and  that  the  trouble  will  "  cure  itself."  Sometimes, 
of  course,  it  does  cure  itself;  but  sometimes  it  does  not; 
and  remediable  disease  has  too  frequently  been  allowed  to 
run  on  in  this  way  until  some  vital  spot  is  attacked  or  the 
trouble  has  become  too  grave  for  medical  skill  to  overcome. 
Many  diseases,  like  a  fire,  may  be  extinguished  at  the  start, 
but  if  not  attended  to,  grow  rapidly  into  a  conflagration 
beyond  control.  Pain  is  one  of  the  most  trustworthy  warn- 
ings that  attention  to  the  mechanism  itself  or  to  our  opera- 
tion of  it  is  necessary;  and  we  have  no  right,  either  for  our 
own  sake  or  that  of  our  friends,  to  neglect  its  warnings. 
While  there  are  times  when  it  is  an  act  of  heroism  to 
endure  suffering  and  to  keep  the  knowledge  of  it  to 
oneself,  there  are  other  times  when  to  do  this  is  not  only 
foolish  but  wrong. 

25.  Hunger  and  Thirst.  —  No  account  of  the  physiology 
of  sensations  would  be  complete  without  some  reference  to 
those  very  common  experiences  of  life,  —  hunger  and  thirst. 
We  have  already  spoken  of  them  as  sensations  which  are 
referred  to  the  body  and  never  to  external  objects,  thirst 
usually  being  referred  to  the  mouth  and  throat,  and  hunger 
frequently  to  the  stomach ;  but  hunger  and  even  thirst 
may  sometimes  affect  us  as  sensations  coming  from  the 


SENSE  OBGANS  265 

body  as  a  whole,  in  which  case  they  are  usually  indistin- 
guishable from  certain  forms  of  general  fatigue. 

It  is  not  yet  fully  understood  how  hunger  and  thirst  are 
excited,  though  it  is  probable  that  they  arise  partly  from 
the  conditions  of  the  stomach  and  throat  respectively,  and 
partly  from  conditions  of  the  body  as  a  whole.  But  they 
are  as  definite  sensations  and  as  truly  adapted  to  guide  us 
in  the  choice  of  food  as  sight  is  adapted  to  picture  to  us 
the  world  in  which  we  live.  So  long  as  the  body  is  normally 
occupied  and  healthy  they  may  usually  be  trusted ;  but 
there  are  abnormal  conditions  of  sedentary  life,  in  the  midst 
of  a  superabundance  of  tempting  food,  when  they  become 
less  trustworthy ;  and  in  some  forms  of  dyspepsia  the  sensa- 
tion of  hunger  is  never  absent,  no  matter  how  often  one 
eats.  In  such  cases  the  very  effort  to  satisfy  hunger  only 
aggravates  disease.  Conditions  of  this  sort  should  not  pre- 
vail if  proper  attention  be  paid  to  the  general  hygienic 
conduct  of  life.  Broadly  speaking,  appetites,  like  fire  and 
dynamite,  are  good  servants  but  bad  masters. 


CHAPTER  XV 

THE  NERVOUS  SYSTEM 

A.  ITS  ANATOMICAL  BASIS 

In  the  preceding  chapter  we  have  repeatedly  emphasized 
the  fact  that  sensations  of  all  kinds  are  developed  in  the 
brain  from  nervous  impulses  coming  from  the  sense  organs ; 

and  in  a  previous 
chapter  (VII)  we 
have  seen  that 
without  reaching 
the  brain,  or  at 
least  without  af- 
fecting conscious- 
ness, these  affer- 
ent impulses  may 
give  rise  to  reflex 
action.  A  reflex 
action  or  a  con- 
scious sensation, 
or  both  a  reflex 
action  and  a  con- 
scious sensation, 
may  therefore 
result  from  the 
FIG.  97.  The  human  brain  viewed  from  above,  entrance  of  a  ner- 
The  cerebral  hemispheres  completely  cover  the  vous  impulse  into 
rest  of  the  brain 

the  central  ner- 
vous system,  and  we  have  now  to  inquire  what  is  known 
of  the  mechanism  by  which  these  results  are  brought  about. 

266 


THE  NEBVOUS  SYSTEM 


267 


For  this  purpose  we  must  first  learn  something  more  of  the 
anatomy  of  the  spinal  cord  and  brain. 

1.  Fundamental  Structure  of  the  Nervous  System.    The 
Brain  of  a  Frog.  —  The  human  spinal  cord  and  brain  are  so 


Forebrain 

'Tweenbrain 
Midbrain 


Hindbrain 


Spinal 
Cord 


FIG.  98.  The  brain  and  spinal  cord  of  the  frog 

On  the  left  is  a  longitudinal,  right  to  left  section,  showing  the  central  canal  and 
the  ventricles  of  the  brain  ;  on  the  right  the  dorsal  view  of  the  brain  and  cord. 
A,  the  cerebral  hemispheres ;  JB,  the  optic  lobes ;  C,  the  cerebellum ;  D,  the 
bulb ;  E,  the  spinal  cord 

complicated  that  it  is  best  to  study  first  the  nervous  system 
of  a  simple  vertebrate  like  the  frog ;  for  the  fundamental 


268 


THE  HUMAN  MECHANISM 


plan  of  structure  is  the  same  in  both.  The  spinal  cord  is 
a  relatively  thick-walled  tube,  the  walls  of  which  are  com- 
posed of  white  and  gray  matter,  the  minute  bore,  or  lumen, 
of  the  tube  being  known  as  the  central  canal.  The  arrange- 
ment in  the  brain  is  similar,  but  here  the  central  space  is 
no  longer  a  small  tube  of  even  bore,  but  consists  for  the 


FIG.  99.  Diagrammatic  median  longitudinal  section  of  a  mammalian  brain. 
After  Edinger 

For  convenience  the  cerebrum  with  its  lateral  ventricle  is  represented  as  a 
single  organ  in  the  median  plane,  instead  of  two  hemispheres  on  either  side 
of  this  plane  and  each  with  its  own  lateral  ventricle.  The  division  into  fore- 
brain,  'tweenbrain,  midbrain,  and  hindbrain  is  marked  by  the  broken  lines 

greater  part  of  irregular  cavities  known  as  the  ventricles  of 
the  brain,  while  the  walls  consist  of  masses  of  gray  and 
white  matter  varying  in  size  and  shape. 

Figure  98  will  assist  the  student  in  understanding  this 
plan  of  structure.  Anteriorly  the  spinal  cord  is  continued 
in  the  bulb  J  whose  central  cavity  is  the  fourth  ventricle. 
Part  of  the  dorsal  wall  of  this  ventricle  forms  the  cere- 
bellum, which  in  the  frog  is  only  slightly  developed,  but 
which  in  higher  vertebrates  (birds  and  mammals)  becomes 
a  large  and  conspicuous  organ.  Anteriorly  the  fourth 
ventricle  is  connected  with  the  third  by  a  tube,  the  aque- 
duct of  Sylvius.  The  thick  walls  of  this  aqueduct  contain 

1  The  older  term  for  the  bulb  is  the  medulla  oblongata,  to  distinguish 
it  from  the  medulla  spinalis,  or  spinal  cord. 


THE  NERVOUS  SYSTEM 


269 


various  masses  of  gray  matter  whose  names  need  not 
detain  us ;  the  Avails  of  the  third  ventricle  are  similarly 
composed  of  large  masses  of  gray  matter  scattered  among 


A 


E 


FIG.  100.  The  base  of  the  human  brain,  showing  the  cranial  nerves 

A,  crus  cerebri,  composed  largely  of  nerve  fibers  which  connect  the  hindbrain 
with  the  'tweenbrain  and  forebrain ;  JB,  pons  Varolii,  the  anterior  floor  of 
the  fourth  ventricle,  connected  laterally  with  the  cerebellum ;  C,  bulb ; 
D,  cerebellum ;  E,  spinal  cord 

the  fibers  of  the  white  matter.  Still  farther  forward  two 
openings  from  the  third  ventricle,  one  on  the  right  and 
one  on  the  left  side,  lead  into  the  large  lateral  ventricles, 
the  nervous  tissue  of  whose  walls  is  the  cerebrum,  or  the 


270 


THE  HUMAN  MECHANISM 


cerebral  hemispheres.  It  is  convenient  to  divide  the  brain 
into  the  fore  brain,  surrounding  the  lateral  ventricles ;  the 
'tweenbrain,  surrounding  the  third  ventricle ;  the  midbrain, 
surrounding  the  aqueduct  of  Sylvius ;  and  the  hindbrain, 
surrounding  the  fourth  ventricle. 

2.  The  Brain  of  the  Mammal  is  built  on  the  same  funda- 
mental plan  as  that  of  the  frog,  and  differs  from  it  mainly 


FIG.  101.  Median  longitudinal  section  of  the  human  brain 

A,  B,  C,  D,  L,  convolutions  of  the  median  surface  of  the  cerebrum ;  E,  F,  the 
cerebellum,  showing  in  the  plane  of  section  the  inner  white  matter  and 
the  outer  gray  matter ;  H,  the  pons  Varolii ;  E,  the  bulb 

in  the  greater  number  of  neurones  and  in  the  complex- 
ity of  their  connections  with  one  another.  This  results  in 
great  thickening  of  the  ventricular  walls  and  the  forma- 
tion of  a  very  complicated  anatomical  structure.  Mammals 
are  especially  characterized  by  an  enormous  development 
of  the  cerebral  hemispheres,  which  in  man  grow  to  such 
proportions  upwards  and  backwards  as  to  overhang  and 
completely  cover  the  other  structures  on  the  dorsal  side. 


THE  NERVOUS  SYSTEM  271 

Bat  even  these  large  masses  of  nervous  tissue,  no  less  than 
the  smaller  cerebrum  of  the  frog,  are  composed  entirely  of 
the  gray  and  white  matter  forming  the  walls  of  the  lateral 
ventricles. 

By  comparing  the  brain  of  a  frog  (Fig.  98)  with  those 
of  the  rabbit,  cat,  and  monkey  (Fig.  146),  and  finally  with 
the  human  brain  (Figs.  97,  100,  101),  a  fairly  good  idea 
may  be  had  of  the  increasing  complexity  of  the  brain  as 
we  pass  from  the  lower  to  the  higher  animals.  Especially 
noteworthy  is  the  greater  relative  prominence  of  the  cere- 
brum. In  the  frog  this  organ  is  small  and  inconspicuous ; 
in  the  rabbit  it"  is  much  larger,  but  its  surface  is  smooth ; 
in  the  cat  there  is  a  further  increase  in  size,  and  the 
surface  is  thrown  into  folds,  or  convolutions ;  and  this  in- 
crease in  size  and  surface  folding  —  carried  yet  farther 
in  the  monkey  —  reaches  its  highest  development  in  the 
human  brain. 

3.  The  Cranial  Nerves.  —  Nerves  enter  the  'tweenbrain, 
midbrain,  and  hindbrain  somewhat  as  they  enter  the  spinal 
cord ;  and,  although  their  separation  into  dorsal  and  ventral 
roots  is  not  obvious,  the  neurones  to  which  their  nerve  fibers 
belong  are  in  all  respects  analogous  to  the  neurones  of  the 
spinal  nerves.    They  may  serve  as  the  paths  of  reflexes 
(e.g.  a  wink  is  a  reflex  from  the  optic  or  the  trigeminal 
nerve  to  the  facial  nerve),  and  their  relation  to  the  cells  of 
the  cerebrum  and  other  higher  portions  of  the  brain  is  essen- 
tially the  same  as  that  of  the  spinal  nerves.    Figure  100 
will  give  t)ie  points  of  entrance  or  exit  of  these  nerves 
from  the  human  brain. 

4.  Histological  Structure  of   the  Brain.  —  Microscopic 
study  of  the  brain  shows  an  aggregation  of  neurones  sim- 
ilar to  that  seen  in  the  spinal  cord.   These  neurones  differ 
greatly  in  shape  (see  Chapter  VII,  p.  74),  in  the  number 
of  their  dendrites,  and  in  the  abundance  of  their  connec- 
tions with  other  neurones.    The  regular  arrangement  in 


272 


THE  HUMAN  MECHANISM 


the  cord  of  central  gray  matter  surrounded  by  white  mat- 
ter is  wanting ;  instead,  masses  of  gray  matter  occur  here 

and  there   among   the 

^^^^^M^^U^^n^^^^,        bundles  of  nerve  fibers 

of  which  the  white  mat- 
ter is  composed.  In  the 
cerebrum  and  cerebel- 
lum the  external  sur- 
face consists  of  gray 
matter  and  is  known 
as  the  cortex  of  the  cere- 
brum and  cerebellum 
respectively.  These 
cortical  structures  form 
the  most  complicated 
system  of  nervous  tis- 
sue in  the  body,  and 
the  cerebral  cortex  is 
intimately  concerned 
with  the  highest  func- 
tions of  the  brain.  (See 
Figs.  102,  103,  and 
104.) 

The    figures    give 
some  idea  of  the  vari- 
ety and  complexity  of 
the  neurones  of  the 
FIG.  102.  A  portion  of  the  gray  matter  (cor-   brain.     But    however 
tex)  of  the  cerebrum  (highly  magnified).     vrc  ,    n          •    •», 

After  Kolliker  dmerent,  at  first  sight, 

Note  the  large  number  of  dendrites.    The  axons    tbe  bmin  ma^  be  fr°m 
are  the  fibers  of  uniform  diameter  running    the    Spinal     COrd,     the 

cen!tVshownlneFfeal9"%  °°*  °'  *""  anatomical  plan  of  or- 

ganization  is  the  same 

in  both  ;  the  brain  as  well  as  the  cord  does  its  work  be- 
cause the  connections  of  its  neurones  with  one  another 


THE  NERVOUS  SYSTEM 


273 


bring  about  coordinated  action.  The  secret  of  the  struc- 
ture of  the  brain,  as  of  the  cord,  lies  in  the  nature 
of  the  connections  of  its  units,  the  neurones,  one  with 
another. 


B.  THE  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM 

Whenever  through  accident,  disease,  or  otherwise,  some 
portion  of  the  nervous  system  is  destroyed,  functions 
dependent  upon 
it  are  no  longer 
performed,  or  at 
least  are  not  per-  A- 
f  ormed  normally. 
A  very  large  B 
number  of  obser- 
vations have  been 
made  upon  both 
animals  and  men 
in  this  condition, 
and  these  have 
made  it  possible 
for  us  to  obtain 
some  idea  of  the 

part  played  in  -plG  10g  Transverse  section  of  a  convolution  of 
normal  life  by  the  cerebellum.  After  Ramon  y  Cajal 

each  part  of  the  The  figure  represents  only  a  few  of  each  kind  of  nerve 
-I  •  1  1  cells  and  nerve  endings.  A,  D,  E,  cells;  B,  C, 

brain     and    COrd.  nerve  endings  (synapses) 

We  shall  attempt 

here  to  sketch  only  a  few  of  the  more  important  outlines 
of  the  picture,  which  the  reader  may  complete  by  more 
extensive  study  of  physiology  and  psychology. 

We  shall  choose  for  study  the  case  of  a  single  animal, 
the  frog,  the  anatomical  structure  of  whose  brain  has  been 
given  in  the  last  chapter.  The  phenomena  shown  by  the 


274 


THE  HUMAN  MECHANISM 


frog  are,  however,  as  far  as  we  shall  describe  them,  in 

general  true  of  higher  vertebrate  animals. 

We  shall  therefore   study  (1)  the   behavior  of  a  frog 

whose  brain  has 
been  destroyed, 
i.e.  a  frog  which 
possesses  no  part 
of  its  central  ner- 
vous system  ex- 
cept the  spinal 
cord  ;  (2)  the  be- 
havior of  a  frog 
with  spinal  cord 
and  bulb  intact, 
but  destitute  of 
midbrain,  'tween- 

Fio,  104.  Section  of  the  cortex  of  the  cerebellum  brain,     and     cere- 
(at  right  angles  to  that  shown  in  Fig.  103).  forum*      (3)      the 

behavior  of  a  frog 


After  Ramon  y  Cajal 


with  spinal  cord,  bulb,  midbrain,  and  'tweenbrain,  but  des- 
titute of  the  cerebrum. 

The  behavior  of  these  incomplete  animals  will  each  be 
compared  with  that  of  a  normal  frog,  which,  of  course,  pos- 
sesses a  complete  nervous  system. 

5.  The  Behavior  of  a  Brainless  Frog,  i.e.  a  frog  which 
possesses  of  its  nervous  system  only  the  spinal  cord.  Such 
a  frog  can  carry  out  only  reflex  actions  of  a  comparatively 
simple  character.  It  lies  flat  upon  its  belly  and,  like  the 
normal  frog,  bends  its  hind  legs  under  its  flank,  but  does 
not  sit  erect  by  supporting  the  head  and  upper  trunk  on  the 
fore  legs.  There  are  no  respiratory  movements  ;  the  vaso- 
constrictor tone  of  the  blood  vessels  is  impaired  or  absent, 
as  are  also  many  other  of  the  most  important  reflexes. 

But  if  one  leg  be  pulled  gently  backward,  the  animal 
will  bend  it  again  to  its  normal  position  under  the  body.  If 


THE  NERVOUS  SYSTEM  275 

the  toe  be  pinched,  the  leg  will  suddenly  be  drawn  away ; 
and  if  the  skin  of  the  flank  be  irritated  by  a  bit  of  filter 
paper  moistened  with  acid,  the  paper  will  be  kicked  off  by 
the  leg  of  the  same  side. 

These  are  all  purposeful1  and  coordinated  actions,  and 
make  upon  the  inexperienced  observer  the  impression  that 
the  frog  is  aware  of  the  stimulus  and  acts  intelligently. 
But  the  mere  fact  that  an  act  is  purposeful  and  coordinated 
does  not  show  that  it  is  a  conscious  act ;  our  movements 
of  respiration,  winking,  coughing,  and  sneezing  are  pur- 
poseful and  coordinated,  but  we  know  well  enough  that 
they,  as  well  as  more  complicated  actions,  may  and  often 
do  occur  in  the  complete  absence  of  consciousness.  One 
of  the  first  lessons  that  the  student  of  animal  behavior 
must  learn  is  not  to  make  the  mistake  of  regarding  an 
action  as  conscious  merely  because  "  it  looks  so,"  or  is 
purposeful  and  more  or  less  highly  coordinated. 

The  spinal  cord  alone,  then,  and  without  the  help  of  the 
brain,  is  capable  of  maintaining  a  small  part  of  the  normal 
posture  of  the  resting  frog,  and  also  of  executing  some  of 
the  simple  reflexes,  especially  those  involving  movements 
of  the  hind  legs  ;  but  it  does  not  seem  to  be  capable  of  origi- 
nating actions  or  of  doing  any  except  reflex  actions. 

6.  The  Behavior  of  a  Frog  with  Spinal  Cord  and  Bulb 
only.  —  In  this  case  there  is  no  new  feature  in  the  main- 
tenance of  posture  ;  the  frog  lies  on  its  belly  and  executes 
the  same  reflexes  as  before.  The  respiratory  movements, 
however,  go  on  in  a  normal  manner ;  the  vasomotor  tone 
of  the  arteries  is  maintained,  most  vasomotor  reflexes  may 
be  produced  with  ease,  and  the  heart  may  be  reflexly  in- 
hibited. As  compared  with  the  brainless  frog,  the  number 
of  actions  which  the  animal  can  execute  is  increased,  and 
the  reflex  movements  become  somewhat  more  complicated; 

1  The  word  purposeful  is  used  here  in  the  same  sense  as  in  Chapter  VII 
(p.  71)  and  does  not  include  conscious  purpose  in  its  meaning. 


276  THE  HUMAN  MECHANISM 

but  the  differences  are  slight,  as  compared  with  those  seen 
in  the  animal  which  has  the  'tweenbrain  and  midbrain  in 
addition  to  the  hindbrain  and  cord. 

7.  The  Behavior  of  a  Frog  with  Spinal  Cord,  Bulb, 
Midbrain,  and  'Tweenbrain ;  that  is  to  say,  a  frog  with 
the  entire  nervous  system  exclusive  of  the  forebrain,  or 
cerebrum.  The  following  points  are  especially  noteworthy : 
(a)  the  sitting  posture  maintained  at  rest ;  (5)  balancing 
movements ;  and  (c)  more  complicated  movements  of 
locomotion. 

(a)  Such  a  frog,  unlike  those  already  described,  sits  erect 
exactly  like  a  normal  frog  ;  and  this  fact  shows  that  com- 
plete maintenance  of  the  normal  posture  requires  the  coop- 
eration of  higher  portions  of  the  nervous  system  than  the 
bulb  and  spinal  cord,  but  does  not  involve  the  cooperation 
of  the  cerebrum. 

(b)  If  the  frog  be  placed  on  a  rectangular  block  of  wood, 
and  the  block  slowly  turned  so  that  the  frog  tends  to  slip 
off  backwards,  it  will  crawl  up  and  over  the  descending 
edge,  keeping  itself  perfectly  balanced.    By  continuing 
to  turn  the  block  the  frog  can  be  made  to  creep  around  it 
almost  indefinitely.    Thus  it  not  only  maintains  the  erect 
position  but  also  corrects  loss  of  equilibrium  by  appropriate 
balancing  movements. 

(c)  If  the  frog  be  stroked  upon  its  belly,  it  will  croak ;  if 
its  lips  be  touched  with  a  blunt  pin,  it  will  brush   the 
pin  away  with  its  forefoot.    Most  important  of  all,  if  it  be 
thrown  into  the  water,  it  will  swim  ;  and  when  it  reaches  a 
solid  object  it  will  crawl  out  upon  it  and  come  to  rest.    In 
short,  the  animal  will  carry  out  almost  any  movement  of 
which  a  normal  frog  is  capable,  provided  the  proper  stim- 
ulus is  applied;  but  without  this  it  will  do  nothing,  though 
capable  of  doing  so  much. 

The  facts  thus  far  brought  forward  show  that  the  neu- 
rones of  the  'tween-,  mid-,  and  hindbrains,  and  of  the  spinal 


THE  NERVOUS  SYSTEM  277 

Cord,  constitute  nervous  mechanisms  which  can  maintain 
the  normal  posture,  correct  loss  of  balance,  and  even  carry 
out  the  usual  acts  of  locomotion.  The  more  of  the  nervous 
system  which  the  animal  retains  the  more  complicated  are 
the  movements,  as  we  should  expect  when  we  remember 
the  increase  in  the  number  of  neurones,  and  the  greater 
complexity  of  coordination  thereby  rendered  possible. 

8.  Comparison  with  the  Normal  Frog.  —  The  behavior 
of  a  frog  lacking  only  the  forebrain  (or  cerebrum)  differs 
from  that  of  a  normal  frog  in  two  most  significant  respects. 
In  the  first  place,  the  animal  rarely  makes  any  movement 
without  obvious  external  stimulation;  if  protected  from 
drying,  it  will  often  sit  motionless  for  days,  or  even  weeks. 
Such  is  not  the  conduct  of  an  animal  which  is  aware  of 
what  is  going  on  around  it,  or  of  its  own  sensations  or 
feelings,  i.e.  of  a  conscious  animal.  In  the  second  place, 
the  frog  shows  the  most  remarkable  regularity  and  persist- 
ency in  making  repeatedly  the  same  response  to  the  same 
stimulus ;  if  its  lips  be  touched  thirty  times  with  a  blunt 
needle,  it  will  brush  at  the  offending  object  every  time  in 
the  same  way  with  the  same  forefoot.  We  should  certainly 
not  expect  a  conscious  animal  to  do  this ;  for,  after  trying 
one  plan  of  action  a  few  times,  it  would  realize  that  its 
efforts  were  unavailing,  and  would  try  something  else, 
such  as  jumping  away.  This  same  peculiarity  is  met  with 
in  all  animals  deprived  of  the  cerebrum.  They  act  like 
mere  complicated  and  faithful  machines ;  they  do  not  act 
as  if  they  were  thoughtful,  original,  or  wise. 

Especially  striking  is  the  avoidance  of  objects  during 
locomotion.  This  fact  looks  at  first  sight  as  if  the  animal 
were  aware  of  the  presence  of  the  obstacle  in  its  path; 
but  a  dog  without  a  cerebrum,  even  when  it  has  been 
without  food  for  a  day  or  more,  will  go  to  one  side  of  a 
piece  of  meat  and  pass  it  by.  He  acts  as  if  unaware  of  the 
nature  of  the  object,  of  its  use  as  food,  etc.  The  image  of 


278  THE  HUMAN  MECHANISM 

the  piece  of  meat  formed  on  his  retina  seems  to  generate 
nervous  impulses  which  pass  to  the  brain  by  way  of  the 
optic  nerve  and  reflexly  guide  the  movements  of  the  dog ; 
but  these  impulses  do  not  inform  the  animal  of  the  nature 
of  the  object,  and  we  have  no  reason  to  believe  that  the 
dog  is  aware  of  the  existence  of  the  meat. 

When  we  consider  our  own  experience  we  find  that  we 
too,  as  we  walk  along  a  crowded  street,  avoid  objects,  not 
only  without  noticing  them  but  without  even  being  aware 
of  their  presence.  Here  again  the  afferent  impulses  from 
the  retina  pass  to  the  nervous  system  and  reflexly  guide 
our  walking  without  affecting  consciousness  at  all.  And 
the  wonderful  feats  of  somnambulism,  where  the  "eyes  are 
open"  but  "  their  sense  is  shut,"  where  the  sleeper  main- 
tains his  balance  and  avoids  stumbling  in  situations  where 
he  would  almost  inevitably  fall  if  he  were  aware  of  his 
surroundings,  show  how  perfect  is  this  very  complicated 
mechanism  of  locomotion,  which  seems  to  be  complete 
even  in  the  absence  of  the  cerebrum. 

We  are,  indeed,  so  accustomed  to  regard  our  actions  as 
volitional  and  conscious  that  we  rarely  consider  the  large 
part  which  reflexes  from  the  eye,  the  ear,  the  skin,  the 
muscles,  and  the  joints  play  in  guiding  them.  We  will  to 
do  a  certain  thing,  to  walk  to  a  certain  point,  for  example ; 
perhaps  the  first  step  is  a  volitional  act,  but  subsequent 
steps,  the  suiting  of  these  steps  to  slight  unevenn esses  of  the 
path,  the  avoidance  of  many  obstacles,  the  maintenance  of 
the  balance  of  the  body  as  a  whole,  —  for  we  walk  not  only 
with  the  legs  but  with  the  entire  body,  —  all  these  things 
take  place  apart  from  any  exercise  of  the  will,  and,  for 
the  greater  part,  in  the  entire  absence  of  consciousness, 
although  consciousness  may,  of  course,  at  any  time  inter- 
vene. Reflex  actions  thus  play  a  most  important  part  even 
in  the  execution  of  those  movements  which  we  think  of  as 
distinctly  conscious  acts. 


THE  NERVOUS  SYSTEM  279 

9.  Connections  of  the  Cerebrum  with  Lower  Portions  of 
the  Nervous  System.    "The  Way  Out."  —  Granting  that 
the  nervous    events  at    the  basis  of  consciousness   occur 
within  the   cerebrum,  how  do  these  events  influence  the 
muscles,  the  glands,  and  other  organs  which  do  the  bid- 
ding of  the  will  ?    What  is  the  way  out  from  this  seat  of 
consciousness?    This  path  has  already   been  referred  to 
in  Chapter  VII  (p.  82).    Cells  in  the  gray  matter  of  the 
cerebrum  give  off  axons  which  pass  downward  through 
the  structures  of  the  'tween-,  mid-,  and  hindbrain  into  the 
white  matter  of  the  spinal  cord.    These  axons  give   off 
along  their  course  collaterals  which  end  in  arborizations 
around  nerve  cells  of  the  lower  portions  of  the  nervous 
system,  and  by  bringing  groups  of  these  cells  into  coordi- 
nated activity  produce  definite  volitional  movements.    The 
student  should  review  carefully  in  this  connection  what 
has  already  been  said  with  reference  to  these   neurones 
(see  Fig.  145,  v). 

10.  Connections  of  the  Cerebrum  with  Lower  Portions  of 
the  Nervous  System.  "  The  Way  In."  —  The  fact  that  affer- 
ent impulses  from  our  sense  organs  of  sight,  hearing,  etc., 
may  affect  consciousness  indicates  that  there  must  be  some 
connection   between   afferent   neurones   and    the   cerebral 
hemispheres,  since  only  when  the  latter  are  present  does  a 
nervous  impulse  produce  a  conscious  sensation.    The  con- 
nection is  not,  however,  so  direct  as  in  the  case  of  efferent 
impulses.    The  neurone  of  the  dorsal  root  may  be  traced 
as  far  as  the  bulb,  but  no  farther ;  from  this  point  the  im- 
pulse can  find  its  way  to  the  cerebrum  only  by  new  neu- 
rones, and  of  these  it  would  seem  that  there  are  several. 
These  relations  are  indicated  in  Fig.  145,  where  the  efferent 
neurones  are  represented  in  black  and  the  afferent  in  red. 

This  diagram  brings  out  the  fact  of  increasing  complexity 
of  reflexes  as  we  proceed  to  the  more  anterior  portions  of 
the  nervous  system.  In  the  spinal  cord  the  collaterals 


280  THE  HUMAN  MECHANISM 

of  the  afferent  neurone  act  upon  the  efferent  neurones ;  in 
the  structures  of  the  midbrain  and  the  'tweenbrain  the 
afferent  tract  makes  connection  with  more  and  more  com- 
plicated and  extensive  systems  of  these  efferent  neurones 
or  motor  mechanisms.  The  range  of  possible  movement 
is  increased  to  include  most  of  the  usual  actions  of  the 
animal,  and  some  of  these  actions  represent  a  very  high 
degree  of  coordination.  Finally,  in  the  cerebrum  the  highest 
of  all  these  connections  is  made  ;  here  take  place  those 
events  of  whose  nature  we  have  thus  far  been  quite  unable 
to  form  any  conception,  but  which  play  some  part  in  the 
genesis  of  conscious  sensations  and  in  the  closely  related 
dispatch  of  volitional  impulses.  We  can  now  understand 
why  it  is  that  removing  this  highest  portion  of  the  nervous 
system  leaves  untouched  not  only  the  simpler  reflexes  but 
even  the  more  complicated  reflexes  of  locomotion,  of 
swimming,  of  flight,  etc. 

11.  Walking  an  Endless  Chain  of  Reflexes. — The  reflex 
mechanism  of  walking  may  not  be  clear  at  first  sight, 
although  the  facts  we  have  already  given  in  this  chapter 
(p.  278)  leave  no  doubt  that  the  movements  of  locomotion 
are  guided  to  some  extent  by  afferent  impulses. 

In  a  previous  chapter  (p.  262)  it  was  shown  that  afferent 
impulses  from  the  skeletal  muscles,  the  joints,  etc.,  enable 
us  to  form  judgments  of  the  position  of  parts  of  our  body 
with  reference  to  each  other ;  we  have  also  seen  that  affer- 
ent impulses  from  the  semicircular  canals  contribute  to 
this  same  sense  of  position.  ;  The  afferent  neurones,  how- 
ever, which  serve  as  the  path  of  these  impulses  to  the 
cerebrum,  send  off  collaterals  to  the  lower  portions  of 
the  brain,  as  shown  in  Fig.  145,  and  in  this  way  make  the 
reflex  connections  which  execute  the  act  of  locomotion. 

With  each  position  which  the  body  assumes,  a  certain 
combination  of  these  afferent  impulses  passes  into  the 
central  nervous  system,  and  each  position  gives  rise  to  its 


THE  NERVOUS  SYSTEM  281 

own  combination  of  impulses.  It  is  clear  (Fig.  145)  that 
these  impulses,  having  entered  the  central  nervous  system, 
have  before  them  one  or  the  other  or  both  of  two  destina- 
tions :  they  may  proceed  onward  to  the  cerebrum  and 
there  give  rise  to  the  knowledge  of  the  position  of  parts 
of  the  body ;  or  they  may  go  by  way  of  collaterals  to  the 
cerebellum  and  other  lower  nerve  centers,  and  there  influ- 
ence the  efferent  or  motor  mechanisms  of  the  more  com- 
plicated reflex  actions. 

It  is  further  necessary  to  understand  that  the  result  of 
the  arrival  of  the  afferent  impulses  at  the  cerebrum  or  the 
lower  centers  depends  on  the  condition  of  those  portions 
of  the  nervous  system  at  the  time.  Thus  the  impulses 
affect  consciousness  only  when  we  heed  them ;  when  one 
is  sitting  still  and  thinking  of  other  things  they  help  to 
maintain  equilibrium  by  their  influence  on  certain  lower 
nerve  centers  which  control  the  muscles,  but  they  do  not 
stimulate  new  movements ;  finally,  during  locomotion  they 
not  only  maintain  the  balance  of  the  body  but  also  call 
forth  the  next  movement,  because  at  that  time  the  efferent 
motor  mechanism  of  the  lower  centers  is  free  to  act. 

In  walking,  for  example,  as  we  take  the  forward  step 
with  one  foot,  the  combination  of  these  afferent  impulses 
is  constantly  changing;  this  change  of  combination  ob- 
viously changes  the  stimuli  acting  upon  the  efferent  or 
motor  cells,  and  the  movement  progresses,  changing  in 
character  each  instant  until  the  foot  taking  the  step  is 
planted  firmly  on  the  ground  and  the  step  with  the  other 
foot  begun.  Here  again  the  same  kind  of  changing  reflex 
movement  goes  on  until  the  second  step  is  completed  and 
the  parts  of  the  body  are  in  the  same  position  with  refer- 
ence to  one  another  as  when  the  first  step  was  begun  ;  we 
return  in  this  way  to  the  original  combination  of  afferent 
impulses  and  so  begin  again  the  same  cycle  of  movements; 
the  third  step  is  like  the  first.  Locomotion  is  thus  an 


282  THE  HUMAN  MECHANISM 

endless  chain  of  reflexes.  The  character  of  the  act  may  be 
modified  by  new  afferent  impulses  from  the  eye  (as  when 
we  unconsciously  avoid  objects),  or  from  touch  when  we 
tread  upon  some  obstacle ;  or  volition  may  modify  it,  as 
when  we  give  attention  to  what  we  are  doing.  But  so 
long  as  the  character  of  the  path  or  of  surrounding  objects 
remains  the  same  and  volition  does  not  intervene,  the  act 
progresses  as  an  endless  chain  of  reflexes. 

Essentially  the  same  thing  is  true  of  swimming,  of  run- 
ning, and  of  many  other  actions.  They  are  fundamentally 
reflexes  from  the  sense  organs  of  position,  though  their 
character  may  be  modified  either  reflexly,  by  new  afferent 
impulses,  or  volitionally^  by  the  intervention  of  new  impulses 
from  the  cerebrum.  Such  an  act  as  dancing  differs  from 
the  others  only  in  the  greater  extent  to  which  the  volitional 
impulses  play  upon  the  chain  of  reflexes  of  which  the 
action  is  fundamentally  composed. 

12.  Actions  resulting  from  Nervous  Processes  originating 
within  the  Cerebrum.  —  A  very  large  part  of  the  activities 
of  the  body  are  thus  fundamentally  reflex  actions ;  they 
do  not  require  the  aid  of  consciousness  for  their  execu- 
tion. And  it  is  fortunate  for  us  that  this  is  the  case;  one 
has  only  to  imagine  a  human  being  who  has  to  give  his 
attention,  or  "  his  mind,"  as  we  often  say,  to  every  adjust- 
ment of  the  digestive,  respiratory,  and  vascular  systems 
required  to  meet  the  changing  necessities  of  life ;  who 
has  to  keep  his  thoughts  on  every  movement  of  walking 
or  running  ;  who  has  to  be  constantly  on  guard  against 
loss  of  balance  even  when  sitting  still.  Such  a  being  is 
almost  inconceivable ;  he  would  "  go  crazy  "  in  a  single 
day  ;  but  we  can  in  this  way  realize  to  what  extent  the 
reflex  mechanisms  of  the  body  perform  the  menial  offices 
of  life,  leaving  the  mind  free  for  higher  things. 

Speech  is  the  result  of  movements  in  which  the  muscles  of 
respiration  —  those  of  the  larynx,  those  of  the  tongue,  and 


THE  NERVOUS  SYSTEM  283 

those  of  the  lips  —  cooperate  to  produce  articulate  and  intel- 
ligible sound.  The  act  of  writing  also  consists  of  a  series 
of  movements  in  which  the  muscles  of  the  arm  and  hand 
cooperate  to  make  thought  visible ;  performing  on  a  musi- 
cal instrument,  modeling  a  figure  in  clay  or  marble  or 
bronze,  painting  a  picture,  —  all  these  things  occur  to  us 
as  examples  of  movements  which  are  not  fundamentally 
reflex,  and  from  which  the  character  of  an  endless  chain 
is  absent.  Such  are  the  highest  actions  of  the  body,  and 
the  movements  of  which  these  actions  are  made  up  are 
chosen  and  directed  by  the  will. 

These  higher  actions,  like  consciousness,  depend  upon 
the  presence  of  the  forebrain.  When  a  certain  area  of  the 
cerebrum  is  destroyed  by  disease,  the  power  of  speech  is 
lost;  when  another  part  is  destroyed,  the  skilled  use  of 
the  hand  is  lost ;  destruction  of  other  portions  affects  in 
the  same  way  others  of  these  skilled  movements.  In  such 
cases  locomotion,  the  maintenance  of  balance,  the  move- 
ments of  respiration,  etc.,  may  be  and  usually  are  unaffected; 
the  patient  merely  loses  the  power  of  doing  one  or  more 
of  those  things  which,  in  no  sense  reflexes,  involve  the 
selection  of  disconnected  and  to  some  extent  independent 
movements  giving  expression  to  some  original  thought, 
sentiment,  or  idea. 

The  neurones  of  the  cerebrum  and  their  connections 
thus  constitute  nervous  mechanisms  whose  activity  is 
essential  to  consciousness,  —  to  our  seeing,  our  hearing, 
our  smelling,  and,  more  than  this,  to  our  understanding 
of  what  we  see,  or  hear,  or  smell,  —  nervous  mechanisms 
whose  activity  is  also  necessary  to  the  expression  of  our 
thought  in  action.  It  is  because  of  this  fact  that,  when 
the  cerebrum  is  removed,  the  animal  becomes  merely  a 
complicated  reflex  machine,  acting  only  as  it  is  immedi- 
ately stimulated  from  without,  or  by  events  taking  place 
within  its  own  body. 


284  THE  HUMAN  MECHANISM 

13.  Effects  of  Anesthetics  on  the  Nervous  System. — When 
a  person  passes  under  the  influence  of  an  anesthetic  the 
first  function  to  disappear  is  consciousness  ;  the  ether  or 
the  chloroform  first  paralyzes  this  highest  and  most  com- 
plex connection  between  the  afferent  and  the  efferent  sides 
of  the  nervous  system.  In  this  condition  the  patient  may 
groan  and  struggle,  for  he  is  in  somewhat  the  same  state 
as  the  animal  without  cerebral  hemispheres.  The  use  of 
the  surgeon's  knife  will  still  produce  movements ;  respira- 
tion may  be  affected  so  as  to  result  in  groans  and  other 
movements  which  the  inexpert  observer,  perhaps  in  alarm, 
attributes  to  severe  suffering;  and  yet  when  the  patient 
awakes  he  tells  us  he  knew  nothing  of  what  passed  and 
felt  no  pain.  It  is  important  to  realize  that  the  signs  of 
pain  are  never  reliable  evidence  of  its  existence. 

If  the  anesthesia  be  pushed  further,  even  these  more 
complicated  reflexes  disappear.  In  the  ordinary  major 
operations  of  surgery,  the  ether  or  the  chloroform  is  given 
until  it  interrupts  not  only  the  cerebral  connections  between 
the  afferent  and  efferent  paths,  but  also  those  of  the  lower 
portions  of  the  brain ;  it  is  even  administered  until  only 
a  few  reflexes  are  left,  such  as  the  wink  when  the  cornea 
is  touched,  the  contraction  of  the  pupil  when  the  eye  is 
exposed  to  light,  etc.,  —  these  serving  as  useful  tests  of  the 
condition  of  the  patient.  If,  for  example,  the  pupil  no 
longer  contracts  to  light,  it  is  an  indication  that  the  anes- 
thesia is  going  too  far,  —  too  near  the  point  where  the  nerv- 
ous mechanism  of  respiration,  etc.,  will  be  paralyzed.  The 
giving  of  ether  is  then  suspended  until  these  reflexes  are 
again  well  established. 

After  the  operation,  as  the  ether  or  chloroform  is  elimi- 
nated from  the  system,  the  reflexes  return  in  the  reverse 
order ;  and  the  unconscious  movements,  groans,  incoherent, 
or  even  more  or  less  coherent,  talking  (comparable  with 
talking  in  one's  sleep)  are  sometimes  most  harrowing  to 


THE  NERVOUS  SYSTEM  285 

the  feelings  of  those  who  do  not  understand  that  they  are 
all  unconscious  acts.  The  physician  and  nurse  who  remain 
unmoved  may  even  be  wrongly  charged  with  lack  of  feel- 
ing because  they  do  not  waste  sympathy  where  they  know 
there  is  neither  suffering  nor  consciousness. 

14.  Inhibitory  Phenomena  in  the  Nervous  System.  —  We 
have  learned  that  some  nerves  excite  organs  to  activity, 
while  others  diminish  activity  or  abolish  it  altogether 
(p.  158).  The  beat  of  the  heart  is  quickened  by  one  set  of 
nerves  and  slowed  by  another ;  the  circular  muscular  fibers 
of  the  arterioles  are  excited  to  contract  by  vasomotor  nerves, 
their  tonic  constriction  is  paralyzed  or  inhibited  by  vaso- 
dilators, and  many  other  examples  might  be  drawn  from 
the  action  of  neurones  on  peripheral  organs  of  the  body. 

Precisely  the  same  thing  is  true  in  the  brain  and  spinal 
cord.  Afferent  impulses  may  not  only  reflexly  excite 
neurones  to  activity  but  may  also  inhibit  the  existing  or 
threatened  activity  of  other  neurones,  as  when  a  sneeze  is 
stopped  by  biting  the  upper  lip  or  by  pinching  the  nose ; 
or  an  action  may  be  inhibited  by  a  volitional  impulse  from 
the  cerebrum,  as  when  the  breathing  movements  are  volun- 
tarily stopped  for  a  while,  or  when  we  similarly  stop  a 
wink  or  a  sneeze.  These  are  all  examples  of  inhibition, 
not  of  the  skeletal  muscles  concerned  but  of  the  neu- 
rones which  innervate  them, — in  other  words,  of  the  inhibi- 
tion of  one  neurone  by  another. 

It  must  be  understood  that  inhibition  is  as  essential  a 
part  of  the  activity  of  the  nervous  system  as  is  excitation. 
Just  as  the  driver  of  a  team  must  urge  on  one  horse  while 
he  restrains  another,  so  in  all  more  complicated  actions, 
probably  in  all  actions,  reflex  or  volitional,  the  orderly 
movement  is  as  much  the  result  of  holding  one  neurone 
in  check  as  of  stimulating  another  one  to  work,  or  to 
work  harder.  Consciousness  proves  its  presence  most  con- 
clusively by  suppressing  reflexes  which  would  otherwise 


286  THE  HUMAN  MECHANISM 

inevitably  occur,  and  by  bringing  about  new  movements 
to  meet  the  desired  end.  Even  in  the  highest  processes  of 
the  most  highly  organized  of  nervous  systems,  viz.  those 
in  which  human  action  originates,  the  man  reveals  his  char- 
acter and  influences  the  world  around  him  by  what  he  does 
not  do,  —  by  what  he  refrains  from  doing,  sometimes  at  the 
cost  of  severe  struggle  against  impulse,  instinct,  or  passion, 
—  quite  as  much  as  by  what  he  does.  Education,  even, 
has  been  defined  as  the  "training  of  inhibitions  and  the 
control  of  reflexes." 

15.  Use  and  Disuse  as  Factors  in  Individual  Develop- 
ment, Training,  and  Efficiency.  —  When  we  consider  the 
marvelously  complicated  character  of  the  nervous  mechan- 
isms which  control  our  actions,  we  naturally  wonder  how 
this  intricate  machinery  can  be  built,  and  why  it  does  not 
more  frequently  get  out  of  order.  We  cannot  say  that  a 
simple  and  comprehensive  answer  will  not  some  day  be 
given  to  these  questions ;  but  to-day  we  have  no  adequate 
answer  whatever.  The  neurones  with  which  we  must 
work  in  life  are  born  with  us,  but  in  most  cases  efficient 
connections  must  subsequently  be  made  between  them, 
thus  perfecting  the  mechanisms  they  compose ;  and  this 
perfecting  of  the  nervous  machine  comes  with  use.  The 
use  of  a  nervous  mechanism  is  generally  essential  to  its 
proper  development,  just  as  the  use  of  a  muscle  is  essential 
to  its  strength.  If  the  child  never  tried  to  walk,  the  neu- 
rones which  carry  out  the  movements  of  walking  would 
not  develop ;  not  only  do  the  muscles  of  an  arm  strapped 
down  to  the  side  of  the  body  waste  away  and  become 
practically  bands  of  connective  tissue,  but  the  neurones 
concerned  in  the  actions  which  the  arms  should  execute 
degenerate  and  are  ultimately  irreparably  injured. 

Provision  is  made  from  earliest  life  for  the  proper  devel- 
opment of  these  neurones,  and  the  establishment  of  irri- 
table connections  between  them  by  use  ;  out  of  the  first 


THE  NEKVOUS  SYSTEM  287 

aimless  movements  of  the  head  and  eyes  and  hands  and 
legs  of  the  baby  the  simpler  coordinating  nervous  mechan- 
isms are  one  by  one  brought  to  perfection ;  then  comes  the 
training  of  those  reflexes  which  maintain  the  erect  position, 
and  of  those  which  govern  locomotion ;  then  play  comes  in, 
with  its  ceaseless  activity,  increasing  still  further  the  num- 
ber of  movements  which  the  nervous  system  can  make, 
and  correspondingly  enlarging  the  possibility  of  human 
achievement.  As  the  child  grows  older  the  family  calls 
upon  him  to  contribute  some  share  to  its  life  or  support ; 
new  activities,  in  the  shape  of  chores  about  the  house 
or  the  farm,  now  share  with  play  the  work  of  the  nerv- 
ous system  ;  activity  becomes  less  general,  more  special. 
Finally,  the  youth  settles  down  to  some  definite  occupa- 
tion or  pursuit,  and  the  more  strictly  this  is  adhered  to 
the  narrower  becomes  the  range  of  activity  ;  the  more  con- 
stantly a  few  systems  of  neurones  are  used,  the  more 
rarely  are  others  called  into  play. 

16.  The  Physical  Basis  of  Habits.  —  All  this  indelibly 
writes  its  history  in  the  nervous  system.  No  fact  is  more 
significant,  or  of  greater  physical  and  moral  import,  than 
that  the  doing  of  any  act  so  affects  the  connections  of 
neurones  with  one  another  as  to  make  it  easier  to  do  the 
same  act  again  under  the  same  conditions  ;  that  refrain- 
ing from  doing  something  toward  which  we  are  inclined 
similarly  renders  more  easy  the  inhibitory  processes  con- 
cerned when  the  same  conditions  impel  us  toward  it  again. 
We  are  largely  what  we  make  ourselves  by  the  training 
which  our  actions  give  to  the  nervous  system. 

And  what  activity  thus  does  for  the  development  of 
power  it  does  also  for  the  maintenance  of  power.  An 
efficient  nervous  mechanism  of  any  kind  once  acquired 
does  not  remain  efficient  without  use.  The  man  who  has 
developed  a  rugged  constitution  in  colder  climates  and 
then  lives  for  years  in  the  tropics,  constantly  exposed  to 


288  THE  HUMAN  MECHANISM 

warm  climate,  finds  on  return  to  the  home  of  his  youth  that 
the  mechanism  of  heat  regulation  does  not  readily  adjust 
itself  to  cold  damp  winds  and  blizzards  ;  the  athlete  who 
has  learned  to  execute  the  greatest  variety  of  "  tricks  " 
in  the  gymnasium  and  then  settles  down  to  a  sedentary 
life  finds  after  some  years  that  he  is  almost  as  helpless  as 
the  man  who  gave  no  attention  to  such  training.  It  is 
unnecessary  to  multiply  examples.  Efficiency  in  any  direc- 
tion is  the  result  of  continued  use  of  organs,  and  especially 
of  continued  training  of  the  nervous  system.  As  we  fit 
ourselves  to  do  some  few  things,  and  to  do  them  well,  we 
have  not  time  to  conserve  by  use  the  efficiency  of  all  the 
nervous  mechanisms  we  have  acquired  ;  we  must  to  some 
extent  sacrifice  the  more  general  actions  for  those  which 
are  more  special  and  useful.  But  it  must  not  be  forgotten 
that  this  can  be  carried  too  far  ;  that  a  certain  amount  of 
general  activity  is  a  condition  of  healthy  living,  and  that 
one  of  the  problems  of  life  to  solve,  and  to  solve  aright,  is 
how  to  distribute  our  activity  between  the  two.  To  the 
consideration  of  these  questions  we  shall  return  in  our 
study  of  personal  hygiene. 


THE  HUMAN  MECHANISM 
PART  II 

THE  HYGIENE  OF  THE  HUMAN  MECH- 
ANISM AND  THE  SANITATION  OF 
ITS  SUKKOUNDINGS 


CHAPTER  XVI 

• 

INTRODUCTORY 

A.  HYGIENE  AND  SANITATION  :   THE  RIGHT  USE  AND 
PKOPER  CARE  OF  THE  HUMAN  MECHANISM 

In  Part  I  of  this  work  we  have  examined  in  some  detail 
the  normal  structure  and  workings  of  the  human  mechan- 
ism. We  have  now  to  consider  that  care  and  use  of  the 
mechanism  which  constitute  a  wise  and  proper  conduct  of 
life,  to  the  end  that  we  may  possess  and  enjoy  the  greatest 
possible  measure  of  bodily  health,  strength,  and  usefulness. 

The  owner  of  any  valuable  lifeless  mechanism,  such  as 
a  watch,  a  yacht,  or  a  piano,  or  of  any  valuable  living 
mechanism,  such  as  a  prize-winning  dog  or  horse,  gives 
close  attention  to  the  proper  care, '  management,  and  use 
of  his  property ;  and  if,  as  often  happens,  great  sums  are 
invested  in  such  property,  this  care  is  naturally  all  the 
greater.  Comparatively  few  persons  own  such  things,  but 
every  one  of  us  —  every  man  and  woman,  every  boy  and 
girl  —  begins  life  possessing  a  far  more  valuable  mech- 
anism which  we  call  the  human  body.  Some  of  these 
mechanisms  are  of  marvelously  strong  and  perfect  con- 
struction; they  seem  to  be  equal  to  any  amount  of  work 
and  to  suffer  but  little  from  careless  operation  or  unfavor- 
able surroundings.  Many  are  not  so  strong,  but  yet  with 
reasonable  care  give  excellent  service.  Others  require  con- 
stant, intelligent  attention,  without  which  they  readily  get 
out  of  order,  but  with  which  they  do  fair  and  even  good 
work.  Much  of  the  best  work  of  the  world  has  been  done 
by  persons  in  poor  health;  Darwin  never  had  robust 

291 


292  THE  HUMAN  MECHANISM 

health ;  Heine  was  an  invalid  in  his  later  years ;  Milton 
was  blind;  Sir  Walter  Scott  was  always  lame;  Pasteur 
was  partially  paralyzed  during  much  of  his  life.  On  the 
other  hand  many,  originally  robust,  have  not  only  broken 
down  and  .failed  to  do  good  work  for  themselves  and  their 
fellows,  but  have  actually  become  a  burden  to  the  world, 
simply  because  they  have  refused  to  give  to  their  bodies 
the  care  which  they  would  freely  bestow  on  a  watch  or 
a  bicycle. 

The  proper  management  and  operation  of  the  human 
mechanism  requires  not  only  care,  but  intelligent  care.  A 
locomotive  is  intrusted  only  to  an  engineer  who  knows  its 
construction,  who  can  detect  the  evidences  that  something 
is  wrong,  who  knows  how  much  steam  to  apply  at  different 
times,  what  to  do  on  various  grades,  how  to  start  his 
engine  safely,  and  how  to  bring  it  to  rest.  By  lessons  on 
anatomy  and  physiology,  in  Part  I,  we  have  endeavored  to 
impart  to  the  student  the  same  preliminary  knowledge  of 
the  construction  and  workings  of  the  human  mechanism 
which  any  one  intending  to  be  an  engineer  must  have 
of  machinery  before  he  can  master  the  practical  opera- 
tion of  his  engine.  The  chapters  immediately  following  are 
concerned  with  the  proper  care  and  management  of  the 
mechanism  under  the  various  conditions  of  daily  life. 

The  principles  governing  the  proper  care  and  right  use 
of  the  human  mechanism  and  its  surroundings  form  the 
subject-matter  of  hygiene  and  sanitation;  and  practical 
hygiene  and  sanitation  consist  in  the  application  of  the 
principles  of  physiology  and  sanitary  science  to  the  con- 
duct of  physical  life.  Their  object  is  the  preservation  and 
promotion  of  health,  the  prevention  of  premature  death,  and 
the  establishment  and  maintenance  of  the  highest  possible 
working  efficiency  of  the  human  mechanism. 


HYGIENE  AND  SANITATION  293 

• 
B.  HEALTH  AND  DISEASE 

All  things  vary  from  time  to  time  in  respect  to  their  con- 
dition. They  may  be  cold  or  hot,  wet  or  dry.  Machines 
may  be  in  good  condition,  —  in  good  working  order,  —  or 
the  reverse ;  and  the  human  body,  the  human  machine,  is 
no  exception.  It  may  be  cold  or  hot,  wet  or  dry,  in  good 
condition  or  in  bad  condition. 

Health  is  a  condition  of  the  human  mechanism  in  which 
all  the  organs  are  sound  and  in  good  working  order. 
"  Perfect  health  "  is  much  the  same  thing,  since  imperfect 
health  suggests  lack  of  soundness,  i.e.  disease.  "  Robust 
health,"  "  strong  health,"  "  sound  health,"  "  good  health," 
and  all  similar  terms  are  easily  understood  as  signifying 
various  states  or  conditions  of  the  human  mechanism.  The 
common  use  of  such  a  term  as  " broken  health"  testifies 
to  the  popular  recognition  of  the  fact  that  health  is  the 
normal  or  sound  condition  of  the  machine.  The  terms 
"  poor  health,"  "  weak  health,"  "  feeble  health,"  etc.,  are 
obviously  contradictory  in  themselves,  and  really  refer  to 
states  or  conditions  in  which  health  is  either  imperfect  or 
altogether  wanting. 

1.  Degrees  of  Health.  —  There  are,  nevertheless,  degrees 
or  variations  in  health  as  in  other  conditions.    A  stone  may 
be  very  wet  or  very  dry,  or  only  slightly  wet  or  half  dry. 
A  watch  may  be  in  tolerably  good  condition  or  in  excellent 
condition.    Likewise  the  human  body  may  be  in  fair  health 
only,  or  in  excellent  health,  or  in  "  splendid  "  health. 

2.  It  is  the  Aim  of  Hygiene  and  Sanitation  to  secure  the 
Best   Health   Possible   under   Any  Given   Conditions.  - — In 
youth  fair  health  is  not  enough  to  expect,  but  in  extreme 
old  age  anything  beyond  this  may  be  impossible.    Some 
trades  or  industries  are,  from  their  very  nature,  unhygienic 
and  correspondingly  dangerous.    Moreover,  the  degree  of 
health,  as  of  other  things,  is  largely  determined  in  each 


294  THE  HUMAN  MECHANISM 

individual  by  personal  ambition,  resolution,  and  effort. 
Many  persons  go  through  life  on  too  low  a  plane  of  health 
simply  because  they  are  too  unambitious,  too  careless, 
or  too  indolent  to  make  the  effort  needed  to  rise  to  any 
higher  plane.  Without  strong  desire  for  improvement, 
improvement  rarely  comes. 

One  characteristic  of  the  human  body  in  health  is  that 
it  does  its  work  with  ease;  but  when  it  passes  into  an 
unsound  or  abnormal  condition  this  characteristic  tends  to 
disappear,  and  dis-ease,  disturbance,  or  difficulty  in  opera- 
tion takes  its  place.  A  "  jumping  "  toothache,  the  "  sore  " 
throat  of  diphtheria,  the  "  uneasiness  "  of  dyspepsia,  the 
"  pains  "  of  rheumatism,  the  "  racking  "  cough  of  advanced 
consumption,  the  "  splitting  "  headache  of  incipient  typhoid 
fever  are  good  examples. 

Disease  is  a  condition  of  the  human  mechanism  in  which 
one  or  more  of  the  organs  is  unsound  or  abnormal,  or  in 
such  poor  working  order  as  to  interfere  seriously  with  the 
welfare  of  the  entire  mechanism. 

3.  Degrees  of  Disease.  —  We  speak  of  "  mild  disease," 
"severe  disease,"  and  even  of  "malignant  disease,"  using 
phrases  which  testify  to  the  fact  that  there  are  degrees  of 
disease  or  disturbance  of  the  human  mechanism,  no  less  than 
of  its  health,  and  the  attention  which  we  give  to  disease  is 
largely  determined  by  its  severity.  When  we  have  a  cold 
in  the  head  we  regard  the  disease  as  a  trifling  matter  and 
expect  it  to  pass  away  of  itself  in  a  day  or  two.  We  are 
apt  to  summon  a  physician  only  when  we  find  that  we 
have  a  fever,  or  when  some  trouble,  which  at  first  seemed 
of  small  consequence,  "  hangs  on  "  or  seems  to  be  getting 
worse  instead  of  better.  We  are  especially  apt  to  call  in 
the  doctor  when  we  are  actually  suffering  pain  and  are 
unable  to  find  relief,  —  all  of  which  facts  bear  witness  to 
our  practical  recognition  of  the  truth  that  there  are  various 
degrees  of  disease  as  well  as  of  health. 


HYGIENE  AND   SANITATION  295 

4.  Different  Attitudes  assumed  towards  Disease.  —  Vari- 
ous attitudes  are  assumed  by  different  persons  towards 
health  and  disease.  One  attitude,  represented  perhaps  by 
the  practice  of  the  majority  of  people,  is  to  go  about  one's 
work,  whatever  that  may  be,  giving  no  thought  whatever 
either  to  the  maintenance  of  health  or  to  the  avoidance 
of  disease  ;  in  other  words,  to  pay  no  attention  to  the  mech- 
anism and  to  do  nothing  to  keep  it  in  order ;  to  wait 
until  something  happens,  some  breakdown  occurs,  some  dis- 
ease has  clearly  developed,  and  then  hastily  to  take  a  dose 
of  medicine,  or,  finally,  to  call  a  physician.  This  we  may 
call  the  attitude  of  heedlessness. 

A  second  attitude  is  that  of  neglecting  any  active  culti- 
vation of  health,  but  carefully  attempting  to  avoid  those 
things  which  are  liable  to  produce  disease.  In  this  case 
persons  often  give  great  attention  to  the  choice  of  diet, 
to  protection  against  cold,  to  the  purity  of  their  drink- 
ing water,  their  food  supplies,  etc.,  fixing  their  attention 
wholly  on  the  agents  of  disease  and  assuming  that,  if  these 
be  kept  at  a  distance,  the  body  will  take  care  of  itself.  This 
may  be  called  a  half-hygienic  attitude. 

A  third  attitude  —  the  reverse  of  the  second  —  consists 
in  actively  cultivating  abounding  health  by  attention  to 
those  things  which  are  believed  to  build  up  a  strong  con- 
stitution, in  the  belief  that  no  disease  can  attack  a  strong 
and  vigorous  body.  Such  persons  concentrate  attention 
on  health  and  underestimate  the  possibilities  of  succumb- 
ing to  attacks  of  disease.  This  also  is  a  half-hygienic  atti- 
tude, although  in  practice  perhaps  somewhat  safer  than 
the  second ;  very  many,  perhaps  all,  diseases  are  less  likely 
to  appear  in  a  strong  and  vigorous  body  than  in  one  which 
is  not  in  sound  health.  But  if  the  experience  of  the  race 
teaches  anything,  it  is  that  strong  men,  seemingly  in  per- 
fect health,  often  succumb  to  attacks  of  disease.  It  is  not 
safe,  even  for  a  healthy  man,  to  swallow  the  germs  of 


296  THE  HUMAN  MECHANISM 

Asiatic  cholera ;  it  is  not  safe,  even  for  a  healthy  man,  to 
prick  his  finger  with  a  knife  which  has  been  used  in 
lancing  a  boil.  Without  in  the  least  undervaluing  the 
importance  of  maintaining  health  and  physical  vigor  as 
preventives  of  disease,  we  cannot  too  strongly  affirm  that 
these  are  not  absolute  preventives,  that  they  are  not  reli- 
able preventives,  and  that  in  some  cases  they  are  not  pre- 
ventives 1  at  all. 

A  fourth  (and  the  only  right)  attitude  toward  health  and 
disease  is  that  which  actively  seeks  to  maintain  in  the 
mechanism  the  highest  possible  degree  of  health  under 
all  conditions,  and  at  the  same  time  constantly  takes  all 
reasonable  precautions  to  ward  off  attacks  of  the  external 
agents  of  disease.  This  is  the  true  hygienic  attitude,  as 
indicated  by  reason  and  modern  science  ;  and  this  atti- 
tude of  mind  we  shall  endeavor  in  the  following  pages  to 
encourage,  justify,  and  strengthen  in  students  or  readers 
of  this  work. 

C.  THE  THREE  GREAT  FACTORS  or  DISEASE 

5.  The  Three  Great  Factors  of  Disease.  —  Keeping 
always  in  mind  the  truth  that  the  human  body  is  a  machine 
or  mechanism,  and  agreeing  to  regard  any  condition  as 
one  of  dis-ease  in  which  the  body  does  not  do  its  work 
smoothly  or  with  ease,  we  perceive  that  there  are  three 
great  causes  of  disease  of  the  body,  just  as  there  are  three 

1  The  truth  of  this  fact  is  illustrated  when  there  appears  among  a 
people  some  disease  to  which  neither  they  nor  their  ancestors  have  regu- 
larly been  exposed  ;  the  ravages  of  epidemics  of  Asiatic  cholera  in  Europe 
and  America,  and  the  history  of  the  great  plague  in  Europe  in  the  seven- 
teenth century,  or  of  yellow  fever  in  our  own  southern  states,  being  cases 
in  point  (see  Daniel  Defoe,  Journal  of  the  Plague  Year ;  James  Ford 
Rhodes,  History  of  the  United  States,  I,  p.  400).  The  North  American 
Indians,  who  were  presumably  strong  and  healthy,  were  decimated  by 
measles,  —  a  comparatively  mild  disease,  —  when  this  was  brought  among 
them  by  the  early  settlers  of  this  country. 


HYGIENE  AND  SANITATION  297 

chief  causes  of  trouble  in  the  running  of  a  locomotive. 
These  are  (1)  imperfections  in  the  mechanism  itself; 
(2)  unskillful  operation  and  care;  and  (3)  unfavorable  exter- 
nal conditions.  Let  us  consider  carefully  the  part  played 
by  each  of  these  in  the  maintenance  of  health  and  the  pre- 
vention of  disease. 

(a)  Imperfections  in  the  Mechanism.  —  The  wheels  of  an 
engine  may  not  be  perfectly  true,  some  of  its  valves  may 
leak,  some  bearing  may  be  unduly  exposed  to  dust.  So 
is  it  with  the  human  body.  Wonderful  as  is  the  human 
mechanism,  it  is  never  perfect.  A  valve  in  the  heart  may 
leak  and  permit  "  regurgitation  "  of  the  blood  ;  a  defect  in 
the  structure  of  the  spine  may  make  it  hard  to  hold  the 
trunk  in  its  normal  posture  ;  the  glands  of  the  stomach  or 
pancreas  may  be  made  of  poor  material  and  so  secrete  an 
ineffective  digestive  juice ;  in  short,  any  organ  may  be  of 
poor  construction  arid  so  have  imperfect  capacity  for  work. 
Such  constitutional  defects  may  be  born  with  us,  or  they 
may  be  acquired  by  some  accident  or  other  circumstance 
which  leads  to  irreparable  and  permanent  injury.  Where 
they  exist  they  must  be  recognized  and  reckoned  with  in 
what  we  attempt  to  do,  although  their  cure  or  compensa- 
tion is  by  no  means  hopeless.  The  deaf  mute  adapts  him- 
self to  a  lack  of  hearing,  and  in  spite  of  it  communicates 
with  his  fellows ;  and  men  and  women  with  serious  organic 
troubles  may  often  lead  useful  and,  on  the  whole,  healthy 
lives. 

Again,  every  human  body  possesses  as  the  outcome  of  its 
construction  or  constitution  more  or  less  capacity  to  endure 
hardship  and  to  struggle  for  continued  existence.  In  the 
strong  this  capacity,  loosely  called  vital  resistance,  may  be 
very  great,  and  in  the  weak  or  feeble  very  small ;  but  in 
order  that  life  shall  continue  at  all,  every  human  body 
must  have  more  or  less  of  it.  It  is  required  to  withstand 
heat  and  cold,  underfeeding  and  overfeeding,  the  attacks 


298  THE  HUMAN  MECHANISM 

of  parasites,  the  work  and  the  play  of  life,  the  infirmities 
of  age.  If  it  be  very  great,  almost  all  hardships  can  be 
endured,  almost  all  diseases  avoided  or  overcome ;  if  it  be 
very  small,  as  it  often  is  in  old  age,  even  the  grasshopper 
may  become  a  burden. 

As  we  pass  middle  life  and  old  age  creeps  over  us  we 
find  this  power  of  vital  resistance  lessened.  Of  all  people 
who  enter  their  seventieth  year,  a  much  larger  percentage 
die  before  reaching  their  next  birthday  than  is  the  case 
with  those  entering  their  twentieth  year.  This  can  only 
mean  that  the  ability  to  cope  with  unfavorable  conditions 
is  lessened  as  age  advances.  The  body  shows  by  growing 
feebleness  that  it  is  wearing  out,  and  ultimately  succumbs 
to  disease  which  in  earlier  life  would  have  been  a  matter 
of  small  consequence.  Hence  it  follows  that  old  people 
must  reckon  with  a  poorer  constitution,  and  must  give 
greater  care  than  the  young  to  the  bodily  machine. 

(b)  Unskillful  Operation  and  Care.  —  The  most  perfect 
engine  will  behave  badly  in  the  hands  of  an  ignorant,  un- 
skillful engineer  or  fireman.  There  is  a  proper  method  of 
firing,  a  proper  method  of  starting;  and  when  a  grade  is 
to  be  ascended  it  must  be  taken  in  the  proper  way.  When 
these  things  are  not  done  rightly  the  engine  is  very  apt  to 
suffer  damage,  even  to  acquire  structural  or  constitutional 
defects ;  and  in  no  such  case  can  it  be  expected  to  do  its 
best  work,  or  to  do  any  work,  with  perfect  ease.  Human 
life  involves  the  operation  of  a  much  more  delicate  engine 
or  mechanism.  The  human  body  is  a  machine  calculated  to 
do  work,  and  when  we  say  that  it  is  alive,  we  mean  that  it 
makes  use  of  the  potential  energy  of  foods  to  accomplish 
ends  which  no  lifeless  machine  can  accomplish ;  but  it  does 
not  do  this  life-work  without  management  or  operation.  It 
is  the  faithful  servant  of  an  intelligent  will,  and  it  may  be 
worked  or  used  wisely  or  unwisely ,  skillfully  or  unskillfully. 
This  engineering,  management,  direction,  or  operation  of 


HYGIENE  AND  SANITATION  299 

the  human  mechanism  constitutes  the  physical  conduct  of 
life,  and  is  one  of  the  most  fundamental  and  important 
elements  in  the  maintenance  of  health. 

(c)  Unfavorable  External  Conditions.  —  Again,  the  best 
work  of  an  engine  requires  more  than  good  construction 
and  skillful  operation  ;  it  also  requires  favorable  conditions 
and  surroundings.  If  the  road-bed  be  poorly  ballasted  or 
the  rails  rusty  and  uneven  ;  if  the  weather  be  so  cold  as  to 
make  it  impossible  to  keep  up  full  steam  in  the  boiler ;  if 
the  water  tanks  be  not  kept  supplied  with  water,  or  the 
coaling  stations  with  fuel,  poor  work  and  often  actual 
injury  to  the  mechanism  itself  —  constitutional  injury  —  is 
the  result.  Finally,  if  by  chance  a  stone  has  rolled  upon 
the  track,  or  a  signal  has  been  wrongly  set  and  a  collision 
results,  a  good  locomotive  may  be  disabled  or  even  ruined. 

So  with  the  human  mechanism.  Like  all  other  living 
things  it  cannot  continue  its  work  under  certain  external 
conditions.  It  cannot  live  without  food  in  a  desert ;  it  can- 
not endure  exposure  to  extreme  cold  without  protection ; 
it  cannot  keep  sound  in  a  room  with  leaky  gas  fixtures,  or 
in  a  cell  which  admits  no  sunshine,  It  must  have  pure 
drink  and  pure  food,  and  it  must  avoid  exposure  to  the 
contagion  of  diseases  against  which  it  has  no  sure  defense. 

6.  Definition  of  "  Environment. " — We  have  already  used 
and  shall  hereafter  often  use  the  terms  "  environment "  and 
•'  environmental,"  and  a  word  in  explanation  of  them  may 
be  serviceable  at  this  point,  although  they  have  been  defined 
briefly  in  Part  I  and  will  be  further  considered  beyond. 
They  are  used  in  this  work,  as  in  biology  in  general,  for 
that  portion  of  the  universe  not  ourselves,  and  generally  for 
those  portions  of  it  comparatively  near  to  us.  A  man's 
home,  for  example,  is  a  part  of  his  normal  environment, 
and  so  are  his  relatives,  friends,  and  neighbors,  his  horses, 
dogs,  and  pets,  his  clothing,  and  his  surroundings  generally. 
In  the  same  category  come,  however,  more  remote  things, 


300  THE  HUMAN  MECHANISM 

such  as  the  village,  town,  city,  or  state  in  which  he  lives. 
Even  the  sun,  the  intermittent  light  from  which  causes  day 
and  night,  with  their  corresponding  alternate  sleeping  and 
waking,  working  and  resting,  though  so  far  away,  is  a  part 
of  man's  environment.  We  have  referred  in  Part  I  to  the 
blood  and  lymph  as  the  environment  of  the  cells  of  the 
body,  and  such  they  are ;  but  this  internal  environment  (of 
the  cells)  need  not  be  confounded  with  the  external  envi- 
ronment (of  the  body).  This  latter  may  be  concisely  defined 
as  everything  outside  ourselves. 

A  close  relation  always  exists  between  the  body  and  its 
environment,  by  which  the  former  is,  so  to  speak,  in  har- 
mony with  or  adapted  to  the  latter.  Just  as  the  ship  is 
adapted  to  the  sea;  the  punt,  the  shell,  and  the  canoe  to 
quiet  waters ;  the  bicycle  to  good  roads,  —  so  also  man's 
body  reaches  its  best  development  and  does  its  best  work, 
not  in  the  tropics  or  at  the  poles,  but  in  the  more  temperate 
zones  to  which  it  appears  to  be  better  adapted.  A  perfectly 
healthy  organism  might,  perhaps,  be  defined  as  one  perfect 
in  construction  or  constitution,  and  perfectly  adapted  to  its 
environment. 

7.  Man  and  His  Environment.  —  Man  is  absolutely 
dependent  on  a  favorable  environment.  He  cannot,  like 
a  fish,  live  long  in  the  sea ;  or,  like  a  bird,  in  the  air.  He 
cannot  even  make  his  way  to  the  poles  of  the  earth ;  and 
at  an  elevation  of  six  or  seven  miles  in  a  balloon  or  on  a 
mountain  he  perishes.  Fire  quickly  destroys,  and  molten 
iron  or  boiling  water  quickly  kills  him.  A  constant  tem- 
perature of  even  100°  F.  causes  him  great  discomfort,  if 
not  danger.  Intense  light  is  painful  to  him,  intense  noise 
disagreeable.  The  same  thing  is  more  or  less  true  of  other 
animals.  They  flee  from  fire,  shudder  or  cry  at  great  noises, 
are  often  dazzled  by  intense  light. 

But  while  the  environment  thus  narrowly  limits  and 
controls  human  life  and  activities,  man  may,  to  a  great 


HYGIENE  AND  SANITATION  301 

extent,  choose  or  control  his  immediate  environment,  — 
a  power  which  he  shares  with  many  other  animals.  The 
beaver  and  the  muskrat  build  rough  shelters  or  houses, 
many  birds  build  nests,  foxes  dig  burrows,  and  some  birds 
and  other  animals  migrate  from  one  environment  to  another. 
Man  also  seeks  or  makes  for  himself  the  shelter  of  caves, 
huts,  wigwams,  houses,  or  hotels.  These  he  heats  if  cold, 
or  cools  if  hot.  He  lights  them  by  windows  by  day,  and  by 
artificial  light  by  night.  He  provides  for  himself,  instead 
of  the  ground,  couches  to  lie  upon,  chairs  to  sit  upon,  and 
tables  upon  which  to  place  his  food.  All  this  is  a  kind 
of  artificial  adaptation  of  the  environment  to  man,  and,  if 
wisely  done,  is  an  important  aid  to  health,  since  it  tends 
to  secure  for  the  body  that  favorable  environment  which 
is  one  of  the  greatest  factors  of  health.  Man  can,  more- 
over, modify  his  surroundings  at  pleasure  and  make  them 
what  they  should  be,  or  at  least  more  nearly  what  they 
should  be.  Fires  are  used  in  winter,  shades  and  fans  in 
summer;  ventilation  replaces  bad  air  with  good  air;  by 
proper  drainage,  filth  and  dampness  are  avoided  ;  by  arti- 
ficial lighting,  darkness  is  done  away  with ;  by  cultivating, 
the  soil  and  by  raising  cereals  and  cattle,  the  food  supply 
is  maintained.  Thus  in  many  ways  the  environment  is 
controlled  and  healthful  conditions  are  promoted  or  main- 
tained. Failing  all  else,  man  may  travel  for  a  while  and 
find  benefit  in  temporary  change ;  or,  like  thousands  arriv- 
ing in  America,  may  abandon  one  environment  and  emigrate 
to  another. 

8.  Scope  and  Subdivisions  of  Hygiene  and  Sanitation.  - 
The  considerations  dwelt  upon  in  the  foregoing  pages  indi- 
cate the  scope  and  possibilities  of  the  science  of  hygiene. 
Given  the  constitution  of  any  individual  as  it  is  at  any  one 
time,  we  must  seek  to  maintain  or  place  that  constitution 
in  a  condition  of  health,  or  efficient  working  order,  in  two 
ways  :  first,  by  the  proper  care  and  operation  of  the  mechanism 


302  THE  HUMAN  MECHANISM 

itself,  including  the  proper  direction  of  its  activities  ;  and 
second,  by  providing  for  it  favorable  surroundings  or  envi- 
ronment. The  former  we  call  hygiene,  the  latter  sanita- 
tion. Each  of  these  efforts  reacts  on  the  constitution; 
improper  operation  of  the  muscles  in  muscular  work,  or 
improper  use  of  the  nervous  system  in  mental  work,  may 
"  undermine "  a  strong  constitution  and  lower  its  vital 
resistance ;  similarly,  a  bad  climate,  a  neglect  of  the  ven- 
tilation of  living  and  sleeping  rooms,  the  use  of  polluted 
water  or  milk  or  other  food,  exposure  to  the  contagion  of 
disease  or  to  excessive  cold  without  proper  clothing,  —  all 
such  failures  to  provide  a  proper  environment  may  injuri- 
ously affect  the  constitution  or  structure  of  the  body. 

It  is  impossible  to  draw  any  sharp  line  between  the  care, 
management,  and  operation  of  the  body  mechanism,  and  the 
care  and  control  of  the  environment,  and  it  is  neither  neces- 
sary nor  desirable  to  do  so  ;  but  we  shall  begin  our  detailed 
study  with  those  things  which  concern  chiefly  the  care,  use, 
and  operation  of  the  mechanism  itself,  —  such,  for  exam- 
ple, as  the  proper  direction  of  muscular  and  nervous  activ- 
ity; alimentation  or  right  feeding ;  the  use  and  abuse  of 
stimulants,  narcotics,  and  other  drugs;  bathing,  clothing,  the 
care  of  the  eyes  and  ears,  etc.  These  matters  which  concern 
chiefly  the  individual  or  the  person  constitute  that  part  of 
our  subject  known  as  personal  hygiene. 

We  shall  then  proceed  to  consider  those  matters  of 
health  which  concern  not  only  individuals  but  communities 
of  individuals,  such  as  families,  cities,  states,  and  nations, — 
for  example,  the  site,  ventilation,  heating,  and  plumbing 
of  the  dwelling  house ;  the  control  of  food  supplies,  as  to 
their  purity ;  public  supplies  of  water  and  milk  ;  sew- 
age disposal ;  the  infectious  and  contagious  diseases.  All 
these  things  require  the  cooperation  of  many  individuals, 
either  as  families,  or  as  citizens  of  an  entire  town,  city,  or 
nation.  Hence  they  are  classed  under  domestic  hygiene  and 


HYGIENE  AND  SANITATION  303 

sanitation  and  public  hygiene  and  sanitation.  The  exact 
meaning  of  these  terms  will  be  explained  in  Chapters 
XXVI  and  XXX. 

It  should  be  understood  that  the  following  statements 
on  the  hygienic  conduct  of  individual  and  social  life  are 
not  equally  applicable  to  all  who  read  them.  For  one, 
those  on  muscular  exercise  are  more  important  than  those 
on  nervous  strain  ;  for  another,  those  on  domestic  hygiene 
are  more  important  than  those  on  the  hygiene  of  the  per- 
son. No  attempt  has  been  made  to  indicate  the  relative 
importance  of  any  part  of  the  subject,  either  by  the  order 
in  which  it  is  treated  or  by  the  amount  of  space  devoted  to 
it.  The  application  must  be  made  by  each  reader  for  him- 
self, with  strict  reference  to  himself  and  to  the  conditions 
of  his  own  life  and  environment.  The  principal  desire 
and  aim  of  the  authors  is  to  persuade  every  one  who 
reads  this  book,  not  merely  to  study  and  to  know  himself 
as  a  physical  mechanism  with  the  same  earnestness  and 
thoroughness  with  which  he  would  study  a  valuable  watch, 
a  bicycle,  a  yacht,  or  an  automobile,  but  also  to  use  that 
mechanism  scientifically — i.e.,  intelligently,  carefully,  and 
skillfully,  — to  the  end  that  life  may  be  longer,  more  use- 
ful, and  more  enjoyable. 


PERSONAL   HYGIENE 

CHAPTER  XVII 

MUSCULAR  ACTIVITY 

* 

A.  THE  MINISTRY  OF  MUSCULAR  ACTIVITY  TO 
THE  BODY  AS  A  WHOLE 

It  is  sufficiently  obvious  that  it  is  through  muscular 
activity  that  we  do  many  necessary,  useful,  or  otherwise 
desirable  things ;  and  it  is  also  a  matter  of  common  expe- 
rience that  muscular  activity  is  required  in  order  to  build 
up  strong  muscles.  A  very  considerable  amount  of  it  is 
required  in  order  that  the  laborer  may  do  his  work,  and  a 
similar  amount  is  necessary  in  order  that  one  may  become 
an  athlete.  But  the  effects  of  muscular  activity  on  the 
body  as  a  whole  are  not  so  obvious ;  while  a  large  num- 
ber of  people  think  that  it  is  "  a  good  thing  "  and  a  smaller 
number  are  convinced  that  it  is  absolutely  necessary  to  the 
best  of  health,  yet  we  not  infrequently  hear  men  and 
women  seriously  question  the  latter  proposition  and  even 
venture  to  doubt  the  truth  of  the  former. 

Now  there  is  nothing  in  hygiene  more  clearly  estab- 
lished than  that  muscular  activity  is  absolutely  essential 
to  healthy  living.  The  effects  of  a  sedentary  life  may  not 
show  themselves  at  once,  but  almost  without  exception  they 
will  assert  themselves  in  the  end.  Muscular  work,  in  other 
words,  not  only  enables  us  to  influence  our  surroundings, 
not  only  builds  up  strong  muscles,  but  in  other  and  equally 
important  though  unseen  ways  ministers  to  the  health  of  the 

304 


MUSCULAR  ACTIVITY  305 

lody  as  a  whole.  It  is  the  purpose  of  this  section  to  present 
this,  the  most  important  hygienic  side  of  our  subject,  by 
describing  some  of  the  physiological  effects  which  muscu- 
lar activity  produces  in  the  body,  and  the  hygienic  value 
of  each  of  these  effects. 

1.  The  Present  Use  of  the  Term  "  Muscular  Activity." 
-  In  the  present  chapter  the  term  "  muscular  activity  "  is 
used  in  a  somewhat  general  sense,  and  without  attempt- 
ing to  set  sharp  limitations  upon  it.  Strictly  speaking, 
of  course,  muscular  activity  would  include  all  work  done 
by  the  muscles  of  the  body,  and  this  is  of  various  kinds. 
Even  those  persons  who  do  no  manual  labor  unconsciously 
perform  muscular  work;  the  heart  works  on,  the  breath 
comes  and  goes  through  orderly  muscular  contractions ; 
sitting  and  standing,  speech,  gestures,  mastication,  —  all 
these  things  involve  muscular  activity,  and  do,  as  a  matter 
of  fact,  contribute  something  to  the  maintenance  of  the 
healthful  conditions  of  the  body.  It  is  not  improbable  that 
they  are  the  physical  salvation  of  thousands  of  people  lead- 
ing sedentary  lives.  At  the  other  extreme  are  those  who 
perform  severe  manual  labor,  or  who  engage  in  vigor- 
ous exercises  or  purposely  cultivate  exceptional  physical 
strength. 

We  are  not,  however,  directly  concerned  at  present  with 
either  of  these  extremes,  nor  with  those  forms  of  muscular 
activity  so  common  to-day  in  workshops  where,  hour  after 
hour,  the  workman  performs  the  same  task  over  and  over 
again.  We  are  rather  concerned  with  those  forms  of  mus- 
cular work  which  are  seen  in  a  lumber  camp  or  on  the 
farm;  which  present  the  characteristic  of  variety  and  in- 
volve the  use  of  the  musculature  of  the  body  as  a  whole ; 
in  short,  those  forms  of  activity  by  which  until  very 
recently  the  human  race  has  supported  itself  in  its  daily 
life.  Such  things  as  brisk  walking,  running,  rowing,  wood- 
chopping,  swimming,  tennis  playing,  would  thus  be  placed 


306  THE  HUMAN  MECHANISM 

in  the  same  class,  since  they  involve  a  use  of  the  muscles 
similar  to  those  which  we  have  mentioned. 

2.  The  Physiological  Effects  of  Muscular  Activity  and 
their  Hygienic  Value.  —  We  may  now  turn  to  the  hygienic 
value  of  the  more  important  physiological  effects  of  these 
general  muscular  activities,  leaving  for  subsequent  consid- 
eration exercises  designed  for  special  purposes,  such  as 
much  of  our  gymnasium  work. 

(a)  The  physical  and  chemical  changes  in  the  working  organ 
are  greater  than  those  accompanying  any  other  bodily  activity. 
The  output  of  carbon  dioxide  by  the  body  per  minute  is 
increased  at  once  from  three-  to  tenfold  with  what  would  be 
termed  moderate  or  vigorous  exertion, while  digestion  seldom 
increases  it  more  than  one  fifth,  and  mental  work  shows 
practically  no  effect  upon  it.    Large  quantities  of  heat  are 
likewise  liberated  and  the  temperature  of  the  muscle  rises 
several  degrees.    These  physical  and  chemical  changes  are 
mentioned  first  because  the  hygienic  effects  upon  the  body 
as  a  whole  are  to  be  traced  to  them  as  the  primary  cause. 

(b)  As  the  result  of  these  changes  in  the  muscles  new 
physical   and   chemical  conditions   are  introduced  into  the 
blood  and  lymph.    The  excess  of  carbon  dioxide  is  entirely 
excreted  by  the  lungs,  so  that  the  blood  carried  to  the 
other  organs  by  the  arteries  shows  no  increase  in  this  sub- 
stance ;  but  other  waste  products  (such  as  salts  of  sarco- 
lactic  acid),  whose  elimination  requires  the  cooperation  of 
other  organs  than  the  lungs,  are  found  in  the  arterial  blood 
in  larger  quantities  than  during  rest.    The  chemical  and 
physical  characteristics  of  the  immediate  environment  of 
every  cell  of  the  body  is  thus  changed,  and  profoundly 
changed.    Let  us  now  consider  the  reaction  of  other  organs 
to  these  changes  in   the  muscles  and  in  the  blood  and 
lymph. 

(c)  Some  of  the  most  striking  effects  of  muscular  work  are 
those  which  are  connected  with  the  heat-regulating  mechanism. 


MUSCULAR  ACTIVITY  307 

The  large  liberation  of  heat  by  the  working  muscle  neces- 
sitates active  measures  to  get  rid  of  that  heat  and  main- 
tain the  constant  temperature.  The  small  arteries  of  the 
skin  dilate,  while  those  of  internal  organs  constrict,  per- 
spiration is  secreted,  and  all  these  processes  are  carried 
out  in  a  coordinated  manner.  The  nervous  mechanism  of 
heat  regulation  is  given  a  new  form  of  activity,  and  thus 
receives  valuable  training  in  adjusting  itself  to  the  chan- 
ging conditions  with  which  it  has  to  cope  in  daily  life. 

(d)  Closely  connected  with  the  foregoing  is  the  (temporary) 
relief  afforded  to   any  congestion   of  blood  in  the  internal 
organs.     Sedentary  occupations  usually  involve  more  or 
less  overfilling  of  the  blood  vessels  of  the  stomach  and 
intestine,  the  pancreas,  the  liver,  the  spleen,  and  the  kid- 
neys ;  they  also  involve  the  absence  of  those  movements 
of  the  trunk  whose  pumping  action  affords  a  marked  assist- 
ance to  the  flow  of  blood  through  the  abdominal  organs 
(p.  147).    The  congestion  thus  caused  is  not  a  good  thing  ; 
it  almost  certainly  renders  the  organs  concerned  more  lia- 
ble to  inflammatory  processes  (see  Chapter  XXI),  and,  if 
there  has  been  established  any  tendency  to  catarrhal  con- 
ditions (see  p.  381),  it  aggravates  that  tendency.    Popular 
experience  has  long  associated  with  health  a  good  color  of 
the  skin ;  and,  while  it  is  not  safe  to  make  such  an  infer- 
ence in  all  cases,  pallor  very  frequently  means  internal  con- 
gestion, unhealthy  digestive  functions,  and  greater  liability 
to  cold  in  the  head  or  the  chest. 

(e)  Muscular   activity  is    the    only    thing  which    can    be 
depended  upon  to  increase  the  work  of  the  heart.    While  this 
fact  makes  caution  and  moderation  necessary  for  persons 
having  certain   forms   of  heart  disease,  yet   for  the  vast 
majority  of  people  it  is  of  the  greatest  hygienic  importance 
to  accustom  the  heart  to  reasonably  hard  work.    Only  in  this 
way  does  it  receive  the  training  necessary  for  its  proper 
development    and   for   the   maintenance   of   its  strength. 


308  THE  HUMAN  MECHANISM 

Emergencies  will  arise  when  the  heart  is  called  upon  for 
severe  effort,  brief  or  prolonged.  The  familiar  example 
of  the  sudden  "  sprint "  for  a  car  is  a  case  in  point ;  and 
there  are  times,  as  in  pneumonia,  when  the  issue  in  sick- 
ness is  largely  determined  by  the  endurance  of  the  heart. 
In  too  many  such  cases,  if  the  patient  escapes  the  fatal 
issue,  it  is  only  with  a  permanently  weakened  heart. 

It  is  important  not  only  that  the  heart  should  be  kept 
ready  for  emergencies  but  also  that  it  be  kept  in  condition 
for  vigorous  work  as  a  regular  duty  of  daily  life.  One  of 
the  worst  of  "  vicious  circles,"  as  physicians  call  them,  is 
the  acquirement  of  a  weakened  heart  by  abstention  from 
proper  muscular  exertion,  and,  as  a  consequence  of  this 
weakened  heart,  increasing  disinclination  to  exertion  of 
any  kind  whatever.  The  failure  to  take  proper  exercise 
leads  to  deterioration  in  strength  and  endurance  on  the 
part  of  the  heart;  and  this  cardiac  deterioration,  with  the 
resulting  discomfort  of  breathlessness,  leads  in  turn  to 
abstention  from  muscular  activity. 

(f)  Muscular  exercise  is  the  one  agent  which  increases  the 
depth  and  frequency  of  the  respiratory  movements.  The 
hygienic  importance  of  this  does  not  lie  in  the  better  oxi- 
dation of  wastes,  since,  so  far  as  we  have  any  accurate 
knowledge  on  the  subject,  it  would  seem  that  the  processes 
of  respiration  during  sedentary  life  more  than  supply  the 
existing  demands  of  the  tissues  for  oxygen.  The  increased 
respiration  is  rather  of  importance  because  of  the  secondary 
effects  of  the  respiratory  movements  in  promoting  the  flow 
of  blood,  and  especially  the  flow  of  lymph  (see  p.  147). 

It  is  probable  that  the  "freshening  effects"  of  muscu- 
lar exercise  are  to  a  very  large  extent  attributable  to  the 
improved  lymph  circulation  in  the  tissues,  and  this  effect, 
it  will  be  remembered,  is  felt  in  the  immediate  environ- 
ment of  almost  every  cell  in  the  body.  The  suction  action 
of  inspiration  quickens  the  lymph  flow  from  all  organs 


MUSCULAR  ACTIVITY  309 

outside  the  thorax  (p.  173),  and  the  increased  pumping 
action  of  the  respiratory  movements  themselves  aids  the 
lymph  flow  from  the  lungs  and  other  organs  within  the 
thorax.  Waste  products  are  more  completely  removed 
from  the  lymph  spaces  surrounding  all  cells,  and  thus  one 
of  the  most  important  of  fatigue  conditions  is  relieved  (see 
p.  59).  Where  lymphatics  are  subject  to  the  pumping 
action  of  contracting  muscles  and  of  the  alternate  flexion 
and  extension  of  joints,  the  suction  action  of  the  respira- 
tory movements  is  reenforced.  This  pumping  action  espe- 
cially affects  the  lymphatics  of  the  arms  and  legs,  and 
those  of  the  abdominal  cavity  (through  the  action  of  the 
diaphragm  and  the  trunk  movements). 

The  increased  respiratory  movements  also  contribute  to 
greater  mobility  of  the  ribs  and  to  the  better  ventilation 
of  the  lungs.  During  vigorous  exercise  all  lobes  of  the 
lungs  are  used,  and  the  dangers  attendant  upon  disuse  of 
the  apical  lobes  (p.  172)  are  largely  obviated. 

(g]  Moderate  exercise  exerts  a  favorable  effect  upon  the 
digestive  organs,  although  the  precise  action  involved  is 
very  complicated.  Here  also  it  improves  the  lymph  flow, 
thus  promoting  absorption  and  producing  better  conditions 
in  all  digestive  glands  and  in  the  muscular  apparatus  of 
the  digestive  tract ;  it  prevents  continued  congestion  and 
the  unfavorable  attendant  conditions.  It  is  probably  also 
a  direct  stimulus  to  peristalsis,  for  unquestionably  the 
exercises  which  involve  movements  of  the  trunk  often 
prove  a  peculiarly  efficient  remedy  for  constipation. 

The  above  summary  is  very  far  from  a  complete  enumer- 
ation of  the  effects  of  muscular  exercise  upon  the  organ- 
ism, but  it  will  suffice  to  show  how  essential  an  element 
such  exercise  is  in  the  life  of  the  body.  The  training  of  the 
heat-regulating  mechanism,  the  training  of  the  heart,  the 
improved  lymphatic  environment  of  every  cell  resulting 


310  THE  HUMAN  MECHANISM 

from  increased  breathing  movements  and  from  the  pumping 
action  of  mechanical  motion,  the  relief  of  internal  conges- 
tions and  the  favorable  influence  upon  digestive  functions, 
—  all  these  things  are  fundamental  to  healthful  cell  life. 

3.  Muscular  Activity  a  Necessity  for  All.  —  We  often 
hear  of  men  and  women  who  live  to  old  age  and  do 
large  amounts  of  mental  work  with  seemingly  little  or 
no  muscular  activity ;  and  it  is  sometimes  suggested  that 
the  experience  of  these  people  proves  that  exercise  is  un- 
necessary. There  are  also  on  record  a  few  cases  of  men 
who  can  drink  large  quantities  of  whisky  without  getting 
drunk  ;  but  it  will  not  be  contended  that  most  men  can 
do  likewise.  As  to  any  line  of  right  hygienic  conduct 
there  are  some  among  the  hundreds  of  millions  inhabiting 
the  earth  who  can  do  the  reverse  with  impunity ;  but  they 
are  not  to  be  taken  as  safe  guides.  The  cases  are  very 
few  indeed  where  abstinence  from  muscular  activity  per- 
sisted in  as  the  rule  of  life  is  without  disastrous  results ; 
the  bad  effects  do  not  always  come  in  a  day  or  a  week  or 
a  year,  but  sooner  or  later  they  almost  invariably  show  them- 
selves. We  must  never  fail  to  distinguish  carefully  between 
the  immediate  and  remote  effects  of  any  line  of  conduct ;  and 
nowhere  is  this  caution  more  needed  than  in  observing  the 
effects  of  a  sedentary  life,  the  evil  results  of  which,  though 
sometimes  long  postponed,  usually  appear  sooner  or  later. 

Some  muscular  exercise  is  a  hygienic  necessity  for  every 
period  of  life  ;  it  belongs  to  no  one  age.  Youth  is  the  time 
when  athletic  sports,  games,  and  all  kinds  of  activity  are 
most  agreeable,  most  necessary,  and  most  enthusiastically 
pursued.  In  old  age  the  changes  which  take  place  in  the 
arterial  walls  necessitate  caution  as  to  severe  exertion. 
But  these  are  only  the  extremes.  Rarely  indeed  do  we 
meet  with  people  who  would  not  be  benefited  by  a  walk 
of  several  miles  a  day,  at  a  rate  of  three  or  four  miles 
an  hour;  and  it  cannot  be  too  strongly  insisted  that  the 


MUSCULAK  ACTIVITY  311 

inability  to  do  this  with  enjoyment  and  profit  is  in  almost 
every  case  because  the  habit  of  taking  exercise  is  not  kept 
up.  The  heart  is  not  as  strong  as  it  once  was ;  the  con- 
nective-tissue elements  of  the  muscles,  the  ligaments,  etc., 
become  sore  upon  taking  exercise,  not  because  of  any  inev- 
itable "  old-age  change,"  but  because  the  ability  to  do  the 
work  easily  has  not  been  maintained  by  constant  practice. 

It  would  be  amusing,  if  it  were  not  sad,  to  see  how  the 
average  adult  American  will  try  almost  everything  which 
holds  out  the  slightest  promise  of  maintaining  some  sort 
of  health  rather  than  take  muscular  exercise,  —  alcoholic 
drinks  (to  dilate  cutaneous  vessels),  Turkish  baths,  mas- 
sage, patent  medicines,  —  anything  rather  than  a  horse- 
back or  bicycle  ride,  or  a  brisk  walk,  or  some  other  simple 
and  perfect  remedy  which  stands  within  easy  reach.  It 
is  not  to  be  expected  that  when  these  exercises  are  first 
tried  after  years  of  sedentary  life,  they  will  be  enjoyed  ; 
and  too  often  the  man  or  woman,  instead  of  persisting 
patiently,  draws  the  conclusion  that  the  time  for  such 
things  has  gone  and  only  resignation  to  old  age  is  in  order. 
When  young  men  and  women  begin  life,  it  should  be  with 
a  clear  conception  of  the  danger  of  falling  into  habits  of 
muscular  inactivity,  and  with  a  conscious  and  strong  deter- 
mination to  avoid  this  danger. 

4.  The  Conservation  of  the  Enjoyment  of  Muscular 
Activity.  —  Muscular  activity  is  so  necessary  for  health, 
for  the  enjoyment  of  life,  and  for  usefulness,  that  the  ability 
to  take  it  should  be  conserved  at  all  costs.  We  should 
not  only  keep  "  in  practice  "  by  making  it  as  much  a  daily 
habit  as  eating  or  sleeping,  but  we  should  also  avoid  those 
unfavorable  conditions  which  interfere  with  our  enjoyment 
of  it.  Some  will  not  walk  a  step  more  than  necessary 
because,  by  the  use  of  improper  shoes,  they  have  acquired 
deformed  feet,  unable  to  support  the  weight  of  the  body; 
sometimes  a  sunstroke,  following  incautious  exposure  to 


312  THE  HUMAN  MECHANISM 

the  hot  sun,  leaves  the  heat-regulating  mechanism  so 
injured  that  muscular  exertion  except  in  cool  weather 
becomes  unsafe  or  even  dangerous  ;  exposure  to  dampness 
often  brings  rheumatism,  an  almost  insuperable  barrier  to 
pleasurable  movement  of  any  kind;  some  infectious  dis- 
eases leave  their  trace  in  the  form  of  an  incurable  organic 
weakness  which  makes  muscular  activity  inadvisable. 
These  things  should,  of  course,  be  avoided  for  their  own 
sake  ;  they  should  be  avoided  also  because  of  their  seri- 
ous indirect  effects  on  health. 

5.  General  Character  of  the  Most  Useful  Exercises.  — 
To  specify  the  exact  forms  or  amounts  of  muscular 
exercise  advisable  would  take  us  beyond  the  scope  of 
the  present  work.  Here  as  elsewhere  the  student  must 
work  out  his  own  salvation.  In  the  following  chapters  we 
shall  discuss,  as  far  as  possible,  the  characteristics  of  some 
special  exercises ;  for  the  present  a  few  general  sugges- 
tions may  prove  useful.  The  muscular  activity  which 
formed  part  of  the  life  of  our  ancestors  may  be  described 
as  generally  moderate,  though  at  times  vigorous  or  hard ; 
only  exceptionally  did  it  involve  extreme  endurance  or 
great  muscular  strain.  Our  ancestors  were  not,  as  a  rule, 
given  to  "  tugs  of  war,"  or  to  putting  up  heavy  dumb- 
bells, or  to  making  inordinately  long  runs,  or  to  "  giant 
swings  "  in  the  gymnasium  ;  nothing  like  a  hundred-yard 
dash  or  a  four-mile  boat  race  was  a  common  occurrence 
among  them.  Where  work  of  this  kind  had  to  be  done  it 
was  left  to  those  who,  by  reason  of  exceptional  strength, 
were  especially  fitted  for  it ;  mankind  as  a  whole  did  no 
such  work,  and  it  is  not  necessary  (or  even  advisable)  for 
most  of  us. 

Nor  can  it  be  claimed  that  the  cultivation  of  great  mus- 
cular strength  was  a  common  practice.  There  was  a  much 
higher  average  of  strength  than  among  us,  and  we  should 
probably  be  better  off  were  our  average  higher  than  it  is ; 


MUSCULAR  ACTIVITY  313 

but  if  we  can  judge  at  all  from  the  history  of  mankind, 
such  training  as  that  required  to  break  some  college 
strength  test  is  not  demanded  for  hygienic  purposes.  Nor 
does  our  own  experience  tell  a  different  story.  Very  strong 
men  are  no  healthier  nor  longer-lived  than  those  of  only 
average  strength,  and,  in  general,  the  athletic  ideal  is  not 
the  hygienic  ideal.  It  is  not  necessarily  unhygienic,  but  it 
is  not  required  for  purposes  of  health. 

It  is  not  desirable  that  exercise  taken  for  general 
hygienic  purposes  shall  be  unduly  fatiguing.  A  moderate 
amount  of  fatigue  is  not  unwholesome,  since  fatigue  brings 
with  it  the  desire  for  rest ;  nor  is  fatiguing  exercise  neces- 
sarily harmful.  But  exercise  need  not  necessarily  be  of 
this  character ;  and,  in  view  of  the  other  work  of  life,  it  is 
certainly  better  to  avoid  undue  fatigue,  especially  when 
we  cannot  rest  well  afterwards.  A  walk  of  six  or  eight 
miles  will  do  more  good  than  one  of  forty  or  fifty. 

6.  Exercise  for  Women.  —  Muscular  exercise  is  no  less 
essential  to  the  health  of  women  than  of  men.  Fortunately 
the  day  is  past  when  false  standards  misinterpreted  the 
truth  that  woman's  most  natural  sphere  in  life  is  the  home 
to  mean  that,  tied  down  to  the  confining  duties  of  house- 
hold life,  she  should  never  know  the  joy  of  movement, 
except  in  dancing  (and  sometimes  not  even  in  that) ;  arid 
then  proceeded  to  make  sure  of  the  result  by  clothing 
her  in  narrow,  pointed,  high-heeled  shoes,  heavy  skirts, 
and  tight-lacing  corsets.  The  reaction  from  this  state  of 
affairs,  at  times  going  to  the  opposite  and  undesirable  ex- 
treme, has  unhappily  at  times  produced  in  women  exhib- 
itions of  mannishness  which  once  led  a  lady  to  speak  of 
"  that  terrible  thing  called  muscular  exercise."  But  dis- 
gust with  these  grotesque  but  avoidable  consequences 
should  not  be  allowed  to  blind  us  to  the  fact  that  a  reason- 
able enjoyment  of  daily  muscular  activity  is  as  much  a 
necessity  for  women  as  for  men. 


314  THE  HUMAN  MECHANISM 

B.  GENERAL  MUSCULAR  EXERCISE 

The  present  section  deals  with  the  use  of  muscular  activ- 
ity for  its  hygienic  effect  upon  the  body  as  a  whole ;  the 
next  section  with  its  employment  for  special  purposes. 
Exercises  undertaken  for  their  general  good  effect  are 
frequently  spoken  of  as  "hygienic";  the  term  is,  how- 
ever, objectionable,  and  we  shall  speak  of  them  as  general 
muscular  exercises. 

General  muscular  exercise  is  of  hygienic  value  because 
it  produces  the  physiological  results  which  have  been 
enumerated  in  the  preceding  section,  —  results  which  have 
been  shown  to  constitute  essential  elements  of  the  normal 
internal  environment  of  the  cells  of  the  body.  To  review 
the  separate  offices  of  this  ministry  to  the  normal  condi- 
tions of  the  body  : 

1.  General  exercises  should  produce  to  a  considerable 
extent  those  physical  and  chemical  changes  which  accom- 
pany muscular  contraction,  with  the  resulting  effects  upon 
the  physiological  condition  of  the  muscle  itself  and  upon 
the  general  internal  environment,  the  blood  and  lymph. 

2.  They  should  exercise,  and  so  train,  the  heat-regulat- 
ing mechanism. 

3.  They  should  tend  to  relieve  vascular  congestion  in 
internal  organs,  bringing  the  blood  in  larger  quantities  to 
the  skin. 

4.  They  should  afford  training  to  the  heart. 

5.  They  should  increase  the  ventilation  of  the  lungs. 

6.  They  should  increase  the  flow  of  lymph  in  the  lym- 
phatics, and  thereby  improve  the  environmental  conditions 
of  all  the  cells  in  the  body  (see  Chapter  IV). 

7.  They  should  exert  a  favorable    influence  upon  the 
digestive  processes,   promoting  proper  secretion  and  ab- 
sorption, and  tending  to  prevent  unhealthful  conditions 
leading  to  constipation. 


MUSCULAR  ACTIVITY  315 

Such  being  the  physiological  ends  sought  for,  we  may 
conclude,  as  to  the  character  of  such  exercises : 

1.  They  should  consist  of  rhythmic  rather  than  of  sus- 
tained contractions.    These  involve  less  fatigue,  are  more 
enjoyable,  and  especially  facilitate  the  flow  of  blood  and 
lymph. 

2.  They  should  be  vigorous,  somewhat  prolonged,  and 
should  usually  be  continuous.    A  brisk  walk  or  a  run  meets 
most  demands  ;  so  do  bicycling  and  many  games.    The 
strolls  or  saunters  which  are  too  frequently  mistaken  for 
exercise  do  not  meet  the  reasonable   hygienic  demands 
of  the  body ;  they  involve  only  an  insignificant  increase  of 
chemical  activity  in  the  muscles,  they  hardly  affect  res- 
piration, they  do  not  train  the  heart;  in  short,  they  do 
not  produce  adequate  physiological  effects  to  accomplish 
hygienic  ends. 

3.  They  should  involve  considerable  movement  on  the  part 
of  the  trunk  as  well  as  the  limbs.    Many  excellent  forms 
of  exercise,  such  as  bicycling,  are  somewhat  deficient  in 
this  respect.    It  is  not  meant  that  sudden   and  violent 
trunk   movements  are   called  for,   but  that  hygienic  ex- 
ercise should  bring  full  change  and  relief  from  the  con- 
strained positions  of  the  trunk  imposed  by  the  sedentary 
occupations  of  modern  life.    A  vigorous  walk,  with  its  ac- 
companying increase  of  breathing  and  trunk  movements, 
fencing,  and  games  which  involve  the  throwing  and  catch- 
ing of  a  ball  are  especially  good  in  this  respect. 

4.  They  should  be  accompanied  by  full  and  free  respira- 
tion.   The  importance  of  this  requirement  needs  no  com- 
ment.   Constricting  clothing    should  not  be  allowed  to 
interfere,  and,  as  far  as  possible,  the  trunk  should  be  held 
erect,  the  neck  and  shoulders  back,  so  as  to  permit  the 
freest  movement  of  the  upper  ribs. 

5.  It  is    advisable  not  to   confine   oneself  wholly  to  one 
form  of  exercise.    Similar  considerations  to  those  which 


316  THE  HUMAN  MECHANISM 

hold  in  the  choice  of  food  apply  to  some  extent  to  exer- 
cise. At  the  same  time  it  must  be  admitted  that  per- 
fect health  can  frequently  be  maintained  to  old  age  by 
using  only  one  kind  of  general  exercise,  such,  for  example, 
as  walking. 

7.  Considerations  concerning  Fatigue.  —  The  relation  of 
general  exercise  to  fatigue  is  a  matter  of  considerable 
interest  and  also  of  importance  in  correlating  muscular 
work  with  the  other  work  of  life.  Fatigue  of  the  whole 
organism  is  a  very  complicated  matter  and  involves  much 
more  than  the  total  amount  of  chemical  change  in  the 
muscles  and  of  the  resulting  waste  products  in  the  system 
as  a  whole.  We  may,  for  example,  be  made  very  tired  by 
unpleasant  sensations  from  the  joints  and  tendons,  or  by 
walking  in  shoes  which  do  not  permit  free  play  to  the 
bones,  ligaments,  tendons,  and  muscles  of  the  foot;  and 
this  even  when  the  amount  of  muscular  exertion  involved 
may  have  been  slight.  It  is  well  known  that  merely  stand- 
ing still  for  a  time  will  frequently  cause  more  fatigue  than 
will  a  longer  time  spent  in  walking. 

Again,  some  forms  of  exercise  throw  a  relatively  large 
share  of  the  total  work  on  some  muscle  or  small  group  of 
muscles,  while  others  distribute  the  total  work  more  evenly 
over  larger  groups.  Walking  and  running  are  very  unlike 
in  this  respect ;  in  the  former  the  weight  of  the  body  must 
be  lifted  from  the  ground  with  each  step,  —  especially 
when  we  walk  very  erect,  —  by  the  extensor  muscles  of 
the  leg,  and  chiefly  by  the  extensors  of  the  ankle  joint; 
running,  on  the  other  hand,  consists  in  a  continual  falling 
forward  and  the  restoration  of  equilibrium  by  a  more  gen- 
eral action  of  the  muscles  of  the  body  as  a  whole.  A  walk 
of  four  and  a  half  miles  an  hour  is  much  more  fatiguing 
to  a  person  in  good  training  than  a  run  of  four  and  a  half 
miles  an  hour,  because  in  the  former  case  a  few  muscles 
are  thrown  into  very  vigorous  contraction  and  so  give  rise 


MUSCULAB  ACTIVITY  317 

to  severe  local  sensations  of  fatigue,   sometimes  accom- 
panied by  cramps  in  the  muscles. 

8.  Some  Examples  of  General  Exercises :  Cycling.  — 
Bicycle  riding  is  remarkable  for  distributing  the  total 
work  over  large  numbers  of  strong  muscles,  so  that  the 
amount  done  by  each  is  relatively  small  ;  consequently, 
where  there  is  but  little  hill-climbing  or  no  strong  head 
winds,  local  fatigue  is  but  slight,  although  the  total  work 
done  by  the  body  is  considerable.  Actual  measurements 
of  the  carbon  dioxide  excreted  have  shown  that  this  is 
much  greater  per  minute  in  a  ride  of  eight  miles  an  hour 
on  a  smooth  level  track,  than  in  walking  three  and  a  half 
miles  an  hour ;  in  other  words,  the  total  work  is  greater. 
The  well-known  increase  of  perspiration  brought  about 
by  such  moderate  riding  points  in  the  same  direction ;  the 
chemical  changes  in  the  body  are  greater  and  so  is  the 
associated  heat  production  ;  and  yet  any  cyclist  knows 
that  the  conscious  fatigue  of  the  ride  is  as  nothing  com- 
pared with  that  of  the  walk.  Moreover,  in  wheeling  the 
weight  of  the  body  is  not  supported  on  the  feet,  and  we 
are  thus  to  a  large  extent  relieved  from  the  unpleasant 
sensations  produced  by  pressure  and  jar  in  the  ankle  and 
knee  joints.  It  is  a  characteristic  of  moderate  or  even 
fairly  vigorous  bicycle  riding  that  it  produces  a  maximum 
of  chemical  change  with  a  minimum  of  fatigue.  This  is 
of  great  practical  importance.  The  larger  production  of 
carbon  dioxide  involves  deeper  breathing,  and,  as  the 
student  now  well  knows,  increased  work  on  the  part  of 
the  heart. 

Within  proper  limits  this  is,  of  course,  good  for  the 
heart ;  there  is  some  danger,  however,  that  in  the  absence 
of  conscious  fatigue  we  may  throw  upon  that  organ  more 
work  than  is  good  for  it,  and  medical  experience  leaves  no 
doubt  that  many  cases  of  injury  to  the  heart  have  resulted 
from  injudicious  cycling ;  that  is  to  say,  from  "  scorching  " 


318  THE  HUMAN  MECHANISM 

against  strong  head  winds  and  in  "  showing  grit  "  by  refus- 
ing to  get  off  and  walk  up  very  steep  hills.  There  are 
occasions  when  it  is  not  wise  to  be  too  ambitious,  and  when 
44  discretion  is  the  better  part  of  valor." 

9.  Some  Examples  of  General  Exercises :  Games.  —  Some- 
what similar  considerations  apply  to  most  of  our  more 
active  games,  such  as  basket-ball,  football,  tether-ball, 
hockey,  polo,  etc.  They  are  perfectly  safe  for  healthy 
people  when  not  played  more  vigorously  than  the  training 
of  the  heart  justifies ;  the  fact  that  there  is  an  element  of 
danger  in  them  is  no  reason  why  they  should  not  be  used, 
but  it  is  a  very  good  reason  why  they  should  not  be  worked 
to  extremes,  and  especially  why  we  should  be  sure,  from 
competent  medical  advice,  that  there  is  in  those  who  play 
them  no  organic  trouble  to  begin  with,  and  that  players 
are  in  good  training  when  they  play  most  intensely. 

The  choice  of  the  kind  of  muscular  work  and  exercise 
involves  so  many  considerations  other  than  those  which 
are  strictly  physiological  and  hygienic  that  it  is  impossible 
to  give  in  an  elementary  treatise  like  this  any  detailed 
discussion  of  the  special  merits  and  defects  of  each.  We 
often  have  other  aims  in  view  besides  the  purely  hygienic ; 
thus  the  group  games,  such  as  football,  baseball,  basket- 
ball, hockey,  etc.,  train  the  spirit  of  cooperation  and  may 
be  made  useful  means  of  moral  training.  In  camping  in 
the  woods  canoeing  is  not  simply  a  means  of  exercise,  but 
also  a  means  of  transportation ;  and  under  other  conditions 
the  same  thing  is  true  of  horseback  riding,  rowing,  etc. 
Woodchopping,  digging,  porterage,  and  plowing  are  valu- 
able means  of  livelihood.  It  is  believed,  however,  that 
the  principles  here  given  will  help  the  individual  to  form 
a  correct  judgment  as  to  whether  his  work  in  life  supplies 
him  incidentally  or  inevitably  with  the  needed  general 
muscular  activity  for  hygienic  purposes,  and,  if  it  does 
not,  to  plan  to  meet  the  want-  intelligently. 


MUSCULAK  ACTIVITY  319 

The  combination  of  muscular  exercise  with  some  other 
pursuit  is  highly  desirable,  and  when  practicable  often 
simplifies  the  hygienic  conduct  of  life.  But  it  is  nothing 
short  of  a  hygienic  misfortune  to  lose  the  youthful  love  of 
activity  for  its  own  sake.  It  is  well  as  we  grow  older  to 
have  golf,  or  a  horse  to  be  exercised  (!),  or  a  fishing  pre- 
serve in  the  woods,  to  "  take  us  out  in  the  open  air  "  and 
make  us  use  our  muscles.  But  a  human  being  who  is 
dependent  upon  something  of  this  kind  to  drag  him  into 
activity  cuts  a  sorry  figure  from  a  moral  standpoint. 
Man's  highest  distinction  is  the  fact  that  his  actions  may 
arise  so  largely  from  processes  of  psychic  life  within  rather 
than  from  some  immediate  stimulus  from  without.  The 
proper  hygienic  conduct  of  life  involves  moral  fiber  as 
well  as  physical  fiber,  and  this  is  especially  true  of  that 
absolutely  essential  part  of  hygienic  conduct  which  de- 
pends upon  the  use  of  organs  like  the  skeletal  muscles, 
which  are  so  largely  subordinate  to  the  commands  of 
the  will. 

10.  Importance  of  Walking  as  a  Means  of  Exercise. - 
In  their  enthusiasm  for  athletic  games  and  outdoor  sports 
in  youth,  and  for  other  outdoor  activities  in  middle  life,  the 
American  people  are  always  in  danger  of  losing  their  love 
for  the  various  forms  of  walking,  such  as  tramping  and 
mountain  climbing.  Walking  is  the  one  form  of  general 
exercise  for  sound  people  which  can  always  be  had  for  the 
taking.  For  this  reason,  if  for  no  other,  it  should  ever  be  a 
part  of  all  sound  physical  training  to  conserve  the  love  of 
tramping  and  the  ability  to  walk.  Apart  from  the  obvious 
fact  that  it  is  in  this  way  that  we  can  get  closest  to  nature 
and  the  real  beauty  of  the  world  in  which  we  live,  the 
possession  of  the  love  of  the  activity  involved  is  one  of  the 
most  precious  possessions  of  our  hygienic  life.  The  man 
or  woman  who  does  not  keep  and  improve  this  power  by 
use  must  look  forward  to  the  same  fate  as  the  servant  in 


320  THE  HUMAN  MECHANISM 

the  parable  who  hid  his  talent  in  a  napkin,  only  to  have 
it  taken  from  him  in  the  end. 

11.  Fresh  Air  not  a  Substitute  for  Muscular  Activity.  - 
A  word  of  warning  is  needed  against  the  folly  of  sup- 
posing that  fresh  air  is  a  substitute  for  muscular  activity. 
Fresh  air  is  one  of  our  greatest  hygienic  blessings,  and  it 
is  very  desirable  to  live  an  outdoor  life  as  far  as  possible. 
But  too  many  think  that  lounging  in  the  shade,  or  riding 
in  the  open  air  in  an  automobile,  a  carriage,  or  an  electric 
car,  does  for  them  what  muscular  exercise  alone  can  do. 
Especially  as  age  creeps  over  us  and  the  love  of  activity 
wanes  from  its  disuse,  more  and  more  does  the  idea  grow 
upon  us  that  " fresh  air"  is  everything.  To  many  the 
possession  of  a  comfortable  carriage  and  a  pair  of  thorough- 
breds is  a  misfortune.  At  one  of  our  most  beautiful  sum- 
mer resorts  some  one  said  to  a  local  physician,  "  Medical 
practice  at  such  a  place  as  this  must  be  very  unremunera- 
tive."  " By  no  means,"  replied  the  man  of  experience ;  "peo- 
ple come  here  where  they  are  tempted  to  overeat ;  in  the 
place  of  exercise  they  lie  back  on  the  cushions  of  their  car- 
riages while  they  are  driven  about ;  their  adipose  tissue 
increases  rapidly,  and  very  soon  it  is  true  that  to  no  class  of 
people  is  the  doctor  so  absolutely  essential  as  to  them."  The 
student  can  easily  make  the  application  for  himself.  Indi- 
gestion, fatty  degeneration,  insomnia,  loss  of  appetite,  nerv- 
ous prostration,  and  kindred  ills  rarely  come  to  those  who 
labor  with  their  hands  ;  and  these  ills  can  be  largely  pre- 
vented, even  in  those  who  must  engage  in  sedentary  occu- 
pations, by  a  wise  and  intelligent  conduct  of  the  physical 
life,  and  especially  by  the  daily  employment  of  an  hour  or 
so  of  vigorous  general  muscular  activity  properly  corre- 
lated with  the  other  work  of  life. 


MUSCULAR  ACTIVITY  321 

C.  MUSCULAR  EXERCISES  FOR  SPECIAL  PURPOSES. 
CORRECTIVE  WORK.    THE  GYMNASIUM 

The  muscles  may  be  used  not  only  to  produce  those 
general  influences  which  are  necessary  to  the  maintenance 
of  health  but  also  to  produce  desirable  special  effects, 
among  which  the  prevention  and  correction  of  faulty  car- 
riage and  action  are  of  great  importance.  In  considering 
the  use  of  muscular  work  for  this  purpose  our  subject 
naturally  groups  itself  under  two  main  divisions :  first, 
faults  of  form  or  carriage  of  the  body  at  rest, —  in  other 
words,  a  bad  figure;  and  second,  faults  of  handling  the 
body  while  it  is  in  motion,  —  in  other  words,  awkwardness 
or  clumsiness. 

12.  The  Shape  or  "  Figure  "  of  the  Body.  —  The  human 
body  may  be  chiseled  in  marble  or  molded  in  bronze, 
and  the  statue  thus  formed  may  recall  to  the  mind  the 
shape  or  figure  of  the  person  it  represents.  But  the  shape 
of  the  living  body  is  not  rigidly  fixed,  as  is  that  of  the 
statue.  The  bony  skeleton  is  sometimes  called  a  frame- 
work, which  supports  the  muscles,  viscera,  skin,  etc.  While 
this  is  to  some  extent  true,  the  organs  are  not  rigidly  sup- 
ported by  the  skeleton,  as  the  canvas  is  supported  by  the 
poles  and  ropes  which  constitute  the  framework  of  a  tent. 
In  other  words,  the  bones  of  the  skeleton  are  not  rigidly 
joined  together  ;  they  do  not  of  themselves  make  a  self- 
supporting  framework;  the  strong  ligaments  which  pass 
from  one  bone  to  another  simply  limit  or  guide  the  move- 
ment of  the  bones  (p.  16) ;  they  do  not,  strictly  speaking, 
bind  them  together.  If  all  organs  save  the  bones  and  liga- 
ments were  removed,  the  skeleton  would  collapse.  It  is 
itself  held  upright  by  the  muscles,  which  determine  what 
position  the  bones  shall  have  with  regard  to  one  another ; 
and  it  is  more  correct  to  say  that  the  muscles  support  the 
skeleton  than  that  the  skeleton  supports  the  muscles. 


322 


THE  HUMAN  MECHANISM 


-B 


—A 


FIG.  105.  Diagram 
showing  the  action 
of  antagonistic 
muscles  which  keep 
the  body  erect. 
After  Huxley 

Arrows  indicate  the  di- 
rection of  the  pull, 
the  feet  serving  as 
a  fixed  basis  of  sup- 
port. The  muscles 
A,  B,H,  and  C keep 
the  body  from  fall- 
ing forward;  D,  E, 
JF\and  G  keep  it  from 
falling  backward 


13.  Round  Shoulders  as  a  Type  of 
Faulty  Carriage.  Their  Cause.  —  The 
carriage  of  the  shoulders  well  illustrates 
the  closing  statement  of  the  last  para- 
graph. Some  people  have  square,  while 
others  have  sloping,  shoulders ;  in  some 
the  shoulders  are  held  back  so  that  the 
upper  portion  of  the  back  is  approxi- 
mately flat,  while  in  others  they  droop 
forward,  thus  causing  the  upper  chest  to 
be  more  or  less  contracted  and  the  back 
44  round."  To  some  extent  these  differ- 
ences maybe  due  to  hereditary  structure ; 
but  they  result,  for  the  most  part,  from 
causes  which  are  largely  if  not  entirely 
under  individual  control.  There  is  little 
or  no  excuse  for  round  shoulders  in 
healthy  people,  and  the  marked  effect  of 
training  is  evident  in  the  fine  bearing  of 
well-trained  soldiers.  The  truth  of  this 
statement  is  seen  when  we  consider  how 
the  deformity  is  usually  acquired,  the 
chief  causes  being  the  following. 

(a)  Faulty  Posture.  —  Round  shoul- 
ders are  uncommon  among  people  whose 
work  requires  an  erect  carriage  of  the 
body ;  for  example,  among  those ,  who 
carry  things  upon  the  head.  With  most, 
however,  the  occupations  of  daily  life 
lead  to  bending  forward  over  work ; 
writing,  drawing,  sewing,  lifting,  gar- 
dening, paving,  machine  and  tool  work 
at  once  occur  as  examples.  The  trunk 
is  held  in  such  a  position  that  the 
shoulders  tend  to  fall  forward  of  their 


MUSCULAR  ACTIVITY 


323 


own  weight.    This  tendency  is  aided  by  the  wrongly  curved 
backs  of  most  chairs,  —  which  seem  as  if  planned  especially 


FIG.  106.  Some  of  the  muscles  of  the  back 

On  the  left  side  are  shown  those  immediately  under  the  skin ;  by  dissecting  away 
this  first  layer,  there  are  exposed  the  muscles  shown  on  the  right  side 

to  force  the  shoulders  forward,  —  and  in  boys  by  the  use 
of  many  forms  of  suspenders. 

(b)  Improper  Balance  in  the  Play  of  Antagonistic  Muscles. 
The  position  of  the  shoulders  with  reference  to  the  ribs, 


324  THE  HUMAN  MECHANISM 

vertebral  column,  and  breastbone  is  largely  dependent  upon 
the  action  of  several  groups  of  antagonistic  muscles,  the 
most  important  of  which  are  those  of  the  breast  and  those 
of  the  back.  Figures  106  and  107  show  the  general  antag- 
onistic action  of  these  muscles.  The  contraction  of  the 
great  breast  (or  pectoral)  muscle  pulls  the  shoulder  for- 
ward and  nearer  the  breastbone  ;  the  contraction  of  the 
back  muscles  (rhomboideus,  trapezius,  and  others)  pulls  them 
backwards  and  nearer  the  backbone.  Both  groups  of  mus- 
cles are  kept  in  a  state  of  sustained  moderate  contraction 
(or  tone)  by  the  nervous  system ;  but  if  the  back  muscles 
relax,  while  those  of  the  pectoral  group  remain  in  tonic 
contraction,  the  shoulder  will  be  pulled  forward  and  the 
back  will  be  round.  Obviously  the  maintenance  by  the 
nervous  system  of  the  proper  balance  in  the  action  of  these 
and  other  antagonistic  groups  of  muscles  is  essential  to 
correct  carriage  of  the  shoulder. 

(c)  Deficient  Use  of  the  Back  Muscles,  with  or  without  the 
Excessive  Use  of  the  Breast  Muscles. — Most  occupations  and 

activities  involve  greater 
use  of  the  breast  muscles 
than  of  the  back  muscles. 
Striking  a  blow  with  a  bat 
or  an  ax,  throwing  a  ball, 
and  similar  actions  are 
more  usual  than  acts,  like 
pulling  taffy,  which  extend 
,  the  arms  and  draw  the 

F,G.  107.  The  skeleton  of  the  trunk   shoulder  blades  closer  to- 
seen  from  above.    After  Demeny       gether.       Movements    of 
Showing  the  antagonistic  play  upon  the    the    first   kind   obviously 

strengthen  the  breast  and 
stretch  the  back  muscles  ; 
those  of  the  second  kind  have  the  opposite  effect.  Conse- 
quently any  marked  preponderance  of  pectoral  action  tends 


MUSCULAR  ACTIVITY  325 

to  elongate  the  back  muscles ;  and  unless  this  is  counter- 
acted by  movements  of  the  opposite  character,  which  stretch 
the  breast  muscle,  the  pectoral  and  back  groups  become 
*'  set,"  as  we  may  express  it,  in  improper  relative  lengths. 


FIG.  108.  Correct  and  incorrect  positions  of  the  shoulder  girdle. 
After  Demeny 

The  result  is  round  shoulders.  Consequently  one  of  the 
most  important  things  to  have  in  view  in  gymnastic  work 
is  the  use  of  movements  which  train  the  back  muscles  and 
stretch  the  pectorals,  thus  counteracting  the  effect  of  the 
one-sided  use  of  these  two  groups  of  muscles  in  ordinary 
occupations. 

14.  The  Period  of  Growth  Especially  Favorable  for  the 
Acquisition  of  Round  Shoulders  and  Other  Deformities.  — 
The  length  of  a  growing  muscle  is  determined  largely  by 
the  distance  between  its  origin  and  insertion  1  during  the 
period  of  growth.  The  breast  muscle  will  grow  to  be  a 
longer  muscle  when  the  shoulders  are  held  back  by  the  back 
muscles  than  when  they  are  habitually  allowed  to  droop 
forward.  In  the  former  case  the  pectorals  grow  to  sufficient 
length  and  do  not  tend  to  pull  the  shoulders  forward  and 
downward ;  and  we  avoid  the  excessive  length  of  the  back 
muscles,  which  makes  it  necessary  for  them  to  take  up  their 
own  slack  before  they  can  keep  the  shoulders  in  position. 

1  Where  a  muscle  is  attached  by  its  two  tendons,  the  point  of  attach- 
ment against  which  it  usually  pulls  or  is  fixed  is  known  as  its  origin, 
while  the  one  it  usually  moves  is  known  as  its  insertion.  Thus  the  origin 
of  the  pectoral  muscle  is  the  breastbone  and  ribs,  its  insertion  the  shoulder 
and  the  upper  arm. 


326  THE  HUMAN  MECHANISM 

The  student  can  now  appreciate  the  fact  that  it  is  in 
youth,  during  the  period  of  growth,  that  deformities  are 
most  readily  acquired  and  most  easily  corrected;  for  the 
muscles,  the  ligaments,  the  bones,  are  then  in  their  forma- 
tive stage.  In  the  case  in  point,  if  the  boy  or  girl  holds 
the  shoulders  properly,  the  pectoral  and  back  muscles  of 
each  side  adjust  themselves  to  their  proper  length ;  and  the 
shoulders  grow  into  the  correct  form,  just  as  the  sapling 
which  is  not  bent  nor  deprived  of  proper  sunlight  grows 
into  the  symmetrical,  beautiful  tree.  During  the  period  of 
growth,  then,  —  say  up  to  at  least  the  twentieth  year,  — 
we  can  hope  to  accomplish  most  in  correcting  and  especially 
in  preventing  deformities.  The  correction  and  prevention 
of  round  shoulders  evidently  depends  upon  the  proper 
training  and  use  of  the  muscles  which  play  upon  the 
shoulder;  it  is  therefore  a  legitimate  part  of  gymnastic 
training,  for  gymnastic  training  is  largely  the  art  of  learn- 
ing to  use  the  muscles  properly. 

Where  there  is  a  special  defect  to  remedy  or  prevent, 
special  exercises  are  required.  These  are  of  the  general 
character  of  the  "  setting-up  "  drill  of  the  soldier,  and  in 
the  case  in  point  we  accomplish  our  purpose  by  using 
movements  which  in  the  first  place  stretch  the  pectorals 
and  even  overextend  them;  in  the  second  place,  give  to 
the  back  muscles  the  exercise  which  they  fail  to  get  in 
our  ordinary  occupations,  and  so  bring  up  their  strength, 
their  ability  to  withstand  fatigue  and  to  maintain  the  tonic 
contraction  demanded  of  them  ;  and  which,  in  the  third 
place,  give  us  the  knowledge  of  the  correct  position  of  the 
shoulders. 

15.  Education  of  the  Consciousness  of  Correct  Posture.  — 
In  explanation  of  the  last  point  we  may  say  that  when 
one  habitually  carries  the  shoulders  properly  he  feels  that 
he  is  taking  an   awkward  position  when  he  allows  the 
shoulders  to  droop  ;    on   the   other  hand,   the   man   who 


MUSCULAB  ACTIVITY  327 

habitually  allows  the  shoulders  to  droop  forward  feels  that 
he  is  in  an  unnatural  position  when  he  holds  his  shoulders 
back.  This  is  largely  because  in  the  first  case  the  back 
muscles  and  in  the  second  the  pectorals  must  be  put  on  a 
stretch  ;  it  is  also  due  to  the  fact  that  the  sensations 
derived  from  the  habitual  posture,  whether  it  be  correct 
or  incorrect,  have  impressed  themselves  on  consciousness 
as  signs  of  the  normal  conditions  ;  to  take  any  other  posi- 
tion is  to  experience  the  feeling  of  something  unusual  or 
abnormal. 

We  learn  of  the  position  of  parts  of  our  body  with 
reference  to  one  another  by  sensations  derived  from  the 
muscles,  tendons,  joints,  etc.  (see  Chapter  XIV,  p.  262); 
and  the  sensations  of  position  which  result  when  we 
assume  the  habitual  posture  fix  themselves  in  our  thought 
as  signs  of  the  normal  posture.  Our  practical,  working 
idea  of  normal  posture,  indeed,  is  nothing  more  nor  less 
than  our  experience  of  the  sensations  of  position  resulting 
from  habitual  posture.  The  man  who  never  carries  his 
shoulders  back  really  knows  nothing  of  their  correct  posi- 
tion, because  the  sensations  from  correct  posture  are  lack- 
ing ;  he  knows  no  more  about  them  than  a  man  blind  from 
his  birth  knows  of  the  color  of  a  landscape.  One  of  the 
first  steps  in  correcting  this  and  similar  faults  must  be  to 
experience  the  muscular  sensations  which  come  from  cor- 
rect carriage  ;  and  the  more  frequently  these  sensations  are 
experienced,  the  better  does  the  subject  become  acquainted 
with  them,  the  more  likely  are  they  to  replace  his  erro- 
neous judgment. 

It  is  only  through  the  sense  of  position  that  we  can  hope 
to  acquire  the  practical  working  knowledge  of  correct 
carriage.  What  we  learn  by  reading  about  the  matter  or 
by  looking  at  pictures  or  statues  of  the  correct  figure  is 
of  little  use  ;  for  such  ideas  come  to  us  only  through  the 
eye,  and  we  obviously  cannot  depend  on  our  sense  of  vision 


328  THE  HUMAN  MECHANISM 

to  inform  us  whether  we  are  carrying  ourselves  properly 
or  not.  We  do  not  "  see  ourselves  as  others  see  us  "  ;  gen- 
erally we  do  not  "  see  ourselves "  at  all.  It  is  only  the 
sense  of  position  that  is  capable  of  reminding  us  the  instant 
we  go  wrong ;  and  this  sense  can  be  trained  properly  only 
by  actually  assuming  the  correct  posture. 

16.  The  More  Important  Faults  of  Form  and  Carriage. 
—  We  may  now  pass  to  the  consideration  of  the  more 
important  deformities,  which  it  is  the  aim  of  special  mus- 
cular exercises  to  prevent  or  correct. 

(a)  The  failure  to  hold  the  neck  erect  (allowing  it  to  bend 
forward).  —  This  results  naturally  from  the  fact  that  the 
weight  of  the  head  will  do  this,  pro- 
vided the  tendency  is  not  corrected 
by  the  proper  training  of  the  muscles 
of  the  back  of  the  neck  and  trunk. 
The  position  of  the  head  usually 
taken  in  reading,  sewing,  etc.,  is 
another  cause  of  this  bad  habit. 

(b)  Hound  or  stoop  shoulders.  — 
These  defects  have  already  been  suf- 
ficiently dwelt  upon  (p.  322). 

(c)  Too   great    backward    (dorsal) 

convexity  of  the  spine  in  the  thoracic 
FIG.  109.  The  results  of          .  ..  A 

proper  (1)  and  improper  region,  and  too  great  forward  (ventral) 

(2)  carriage  of  the  ver-  convexity  of  the  spine  in  the  abdominal 

tebral  column.     After  reqion.  —  A  certain  amount  of  such 
Demeny  .  .   .       . 

curvature  is  normal  in  these  regions 

(see  Chapter  II) ;  but  there  is  usually  a  tendency  to  exces- 
sive curvature  because  of  the  weight  of  the  parts  of  the 
body  which  the  spine  must  support.  Every  one  knows 
that  it  is  an  effort  to  sit  erect  ;  and  this  feeling  of  effort 
comes  from  the  fact  that  the  spine  is  straightened,  or  rather 
its  curvature  kept  normal,  by  the  action  of  a  rather  com- 
plicated group  of  muscles,  —  the  erectors  of  the  spine.  To 


MUSCULAR  ACTIVITY  329 

sit,  or  stand,  or  walk  erect  involves  the  activity  of  these 
muscles  ;  when  they  cease  to  act  the  faulty  curvature  be- 
comes more  pronounced.  Hence  the  value  of  all  exercises 
which  tend  to  straighten  the  spine,  —  exercises,  for  exam- 
ple, in  which,  while  standing  on  the  feet,  we  try  by  our 
own  muscular  effort  to  make  ourselves  as  tall  as  possible. 
They  train  and  strengthen  the  muscles  in  question ;  they 
stretch  their  antagonists,  just  as  throwing  the  shoulders 
back  stretches  the  pectorals;  and  they  impart  to  us  by 
actual  experience  the  sensation  of  being  erect. 

(d)  Lateral  curvature  of  the  spine.  —  When  the  spinal 
column  and  its  attached  ligaments  and  muscles  are  prop- 
erly developed  there  is  little  or  no  lateral  curvature  of  the 
spine;  the  two  halves  of  the  body  are  symmetrical  with 
regard  to  the  median  plane  of  the  body,  although  a  con- 
siderable amount  of  bending  of  the  spine  as  a  whole  to 
one  side  or  the  other  is  possible.    It  is,  however,  quite 
possible,  by  maintaining  incorrect  positions,  to  acquire  a 
more  or  less  pronounced  lateral  curvature  in  which  the 
muscles  and  ligaments  of  the  concave  side  become  short- 
ened and  those  of  the  convex  side  lengthened.    Perhaps 
nothing  is  so  responsible  for  all  these  faults  of  curvature 
6f  the  spinal  column  as  improper  positions  at  the  school 
desk,  and  much  can  be  done  to  prevent  them  by  properly 
constructed  school  furniture  and  careful  attention  to  cor- 
rect position.    But  it  is  not  wise  to  depend  on  these  alone. 
No  desk  has  been  constructed  in  which  correct  posture 
can  be  indefinitely  maintained  with  ease,  and  we  have  still 
in  any  case  to  contend  with  the  force  of  gravity.    Active 
exercises  which  straighten  the  spine  should  supplement 
the  other  measures.    Experience  has  well  established  the 
fact  that  the  true  preventive  and  remedy  lies  in  move- 
ments which  elongate  the  spine. 

(e)  We  have   elsewhere   (p.  172)   pointed  out  the  im- 
portant action  of  the  muscles  of  the  abdominal  wall  in 


330  THE  HUMAN  MECHANISM 

supporting  the  abdominal  viscera,  especially  those,  like 
the  stomach,  the  spleen,  and  the  intestine,  which  are  sus- 
pended from  the  dorsal  wall  of  the  abdominal  cavity. 
Fig.  137  will  at  once  make  clear  how  the  relaxation  or 
elongation  of  the  abdominal  muscles,  by  removing  support 
from  these  viscera,  permits  their  weight  to  pull  unduly 
upon  the  mesentery,  and  so  to  stretch  this  support.  It  is 
also  not  improbable  that  the  tense  mesentery  at  times,  by 
pressing  upon  thin-walled  veins  and  lymphatics,  interferes 
with  the  circulation  of  blood  and  the  flow  of  lymph  in 
some  organs,  and  so  leads  to  trouble.  A  pot-belly  is  not 
a  thing  of  beauty,  and  there  is  every  reason  for  thinking 
it  to  be  undesirable  from  the  hygienic  point  of  view.  It 
is  prevented,  in  the  first  place,  by  every  movement  which 
prevents  undue  lumbar  curvature  of  the  spine,  and,  in  the 
second  place,  by  exercises  of  the  abdominal  muscles,  which 
result  in  their  improved  tone.  These,  however,  like  all 
corrective  exercises,  must  be  followed  up  by  maintenance  of 
the  correct  position  of  the  trunk. 

17.  Special  Exercise  for  the  Training  of  Nervous  Coordi- 
nation. —  A  man  or  woman  may  possess  none  of  the  de- 
formities noticed  above,  —  the  anatomical  form  of  the  body 
may  conform  to  the  best  ideals,  —  and  yet  the  positions 
and  movements  of  the  body  may  be  awkward,  inexpert, 
ungraceful.  In  other  words,  the  muscles  may  be  well  devel- 
oped but  the  individual  may  be  deficient  in  the  power  of 
easily  coordinating  their  action  in  the  accomplishment  of 
desired  ends.  After  what  has  been  learned  of  the  part 
which  the  nervous  system  plays  in  directing  our  actions, 
the  brevity  of  any  reference  to  this  purpose  of  physical 
training  will  not  mislead  the  student  into  thinking  that  it 
is  of  little  importance.  We  have  learned  that  the  mainte- 
nance of  equilibrium,  when  the  body  is  at  rest  and  when 
it  is  in  motion,  and  the  execution  of  complicated  move- 
ments, both  require  training  of  the  nervous  system  by  use. 


MUSCULAR  ACTIVITY  331 

The  range  of  activities  for  which  we  can  train  is  very  exten- 
sive ;  playing  upon  musical  instruments,  the  execution  of 
gymnastic  feats  on  the  parallel  and  horizontal  bars,  the 
traveling  rings,  or  the  trapeze,  are  only  a  few  examples  of 
what  can  be  done  by  the  training  of  the  nervous  system 
by  practice. 

A  large  part  of  gymnasium  work  consists  in  this  sort  of 
training,  and  there  is  almost  no  limit  to  the  forms  of  exer- 
cise to  which  we  may  train,  —  vaulting,  jumping,  balan- 
cing the  body  on  one  foot  while  various  movements  are 
made,  the  tricks  of  the  parallel  and  horizontal  bars,  trapeze, 
etc.  Is  there  any  principle  to  guide  us  in  the  choice  of 
what  we  shall  do  ?  In  reply  to  this  question  we  may  say 
that  the  leading  principle  should  undoubtedly  be  that  of 
training  for  what  will  be  useful,  and  while  we  need  not 
discard  all  training  which  cannot  be  justified  on  this 
ground,  that  which  is  useful  should  not  be  sacrificed  to 
that  which  is  not  useful.  A  large  amount  of  skill  is 
required  to  walk  upon  the  hands  with  the  feet  in  the  air, 
and  the  thing  can  be  done  very  gracefully  by  training ; 
but  it  is  certainly  better  to  cultivate  the  habit  of  walking 
gracefully  upon  the  feet.  And  yet  one  may  see  profes- 
sional gymnasts  who  are  extremely  graceful  while  perform- 
ing their  tricks,  but  whose  gait  is  clumsy  and  awkward. 

18.  Balance  Exercises.  —  It  is  evident  that  by  far  the 
greater  number  of  our  customary  coordinated  movements 
are  made  on  the  feet.  Hence  the  value  of  so-called  balance 
exercises  in  the  widest  sense,  whether  they  consist  in 
the  execution  of  difficult  movements  while  standing  on 
one  foot  or  on  the  "  walking  beam,"  or  in  making  a  proper 
landing  from  a  jump  or  a  vault ;  all  of  them  afford  training 
of  those  reflexes  by  which  we  retain  control  of  the  body 
in  motion,  thus  securing  grace  of  posture  and  carriage. 

The  general  purpose  of  training  these  reflexes  is  the 
same  as  the  purpose  of  those  exercises  which  correct 


332  THE  HUMAN  MECHANISM 

deformities ;  they  do  for  the  nervous  mechanism  of  the 
movement  what  the  others  do  for  the  skeletal  parts  and 
the  muscles  which  play  upon  them  ;  they  give  the  training 
of  use  and  prevent  atrophy  from  disuse. 

Both  these  ends,  the  corrective  and  the  so-called  coordi- 
native,  are  best  secured  by  the  use  of  gymnastic  move- 
ments ;  and  the  increasingly  sedentary  character  of  much 
of  our  modern  life  correspondingly  increases  the  value  of 
gymnastic  work,  especially  in  the  period  of  youth.  It  is 
well  to  learn  and  understand  the  most  useful  exercises, 
and  even  in  adult  life  to  have  resort  to  them  two  or  three 
times  a  week  in  order  to  hold  fast  what  has  been  gained. 

19.  The  Gymnasium  as  a  Means  of  General  Muscular 
Exercise.  —  Under  the  conditions  of  city  life,  especially  in 
winter  time,  the  gymnasium  is  also  useful  in  supplying 
general  exercise  in  the  form  of  running,  gymnastic  games, 
etc.    It  is  better  to  seek  outdoor  work  as  far  as  possible 
for  this  exercise,  but  there  are  times  when  those  living  in 
the  heart  of  crowded  cities  cannot  get  to  the  country,  and 
outdoor  exercise  in  town  is  not  all  that  is  to  be  desired. 
While  there  is  sometimes  a  tendency  to  extol  unduly  the 
value  of  gymnastic  work,  there  is  equally  marked  igno- 
rance in  other  quarters   as  to  what  the  gymnasium  may 
accomplish.    Our  cities  are  vastly  better  off  for  their  Y.  M. 
C.  A.  and  other  gymnasia,  and  we  cannot  afford  to  discour- 
age any  means  of  properly  directed  physical  training. 

20.  Hygienic  Value  of  Corrective  Work.  —  Before  leaving 
the  subject  of  corrective  and  coordinative  work  we  may 
answer  a  question  which  is  frequently  asked  :   Has  it,  after 
all,  any  hygienic  value?    All  will  readily  grant  that  this 
part  of  physical  training  has  an  aesthetic  value,  and  that 
the  cultivation  of  the  taste  for  correct  form  and  carriage 
in    one's    own    person    is    to    be    commended.     But   is    a 
man  less  healthy  for  being  round-shouldered  ?    The  answer 
is  that  he  may  or  may  not  be  less  healthy.    The  deformity 


MUSCULAR  ACTIVITY  333 

of  round  shoulders  carries  with  it  the  lessened  use  of 
the  upper  ribs  in  breathing ;  and  while  one  man  or  woman 
may  escape  dangerous  consequences,  another  may  not,— 
indeed  we  know  does  not,  and  it  is  the  part  of  wisdom 
to  avoid  the  danger  as  far  as  possible.  In  one  a  pot-belly 
may  be  consistent  with  perfect  health,  while  in  others  it  is 
not.  One  may  go  through  life  with  some  faulty  curvature 
or  the  spine  and  not  suffer  from  it ;  but  thousands  of  per- 
sons have  to  consult  physicians  every  year  because  of  such 
faults.  Many  a  man  wears  improper  shoes  without  bad 
results  ;  hundreds  pay  for  it  with  flat  foot  and  suffering 
which  at  times  amounts  to  torture.  There  is  not  a  single 
deformity  enumerated  above  which  may  not  prove  a  seri- 
ous matter  ;  and  when  it  is  so  easy  to  avoid  most  of  them, 
it  would  seem  from  a  hygienic  point  of  view  well  worth 
while  to  do  so. 

The  hygienic  value  of  corrective  and  coordinative  work 
is  justified,  however,  still  more  effectively  on  another 
ground.  The  tendency  to  take  general  exercise  is  directly 
proportional  to  the  excellence  of  the  neuromuscular 
mechanism  of  the  body.  The  man  who  is  awkward  and 
clumsy,  who  can  make  but  few  movements,  does  not  enjoy 
general  exercise  as  does  the  man  who  has  good  control  of 
his  muscles  and  can  make  many  movements.  It  is  proba- 
bly not  too  much  to  say  that  a  very  large  proportion  of  the 
people  who  settle  down  to  a  sedentary  life  with  the  coming 
of  their  thirty-fifth  or  fortieth  year  do  this  because  they  can 
do  so  little  with  the  body,  and  because  exercise  is  conse- 
quently monotonous  and  distasteful.  We  can  undoubtedly 
preserve  more  readily  the  love  of  movement  for  its  own 
sake  when  we  have  a  body  which  can  move  freely  and 
easily,  skillfully  and  joyously,  than  when  we  have  one 
which  is  never  so  much  at  home  as  in  an  easy-chair  or 
upon  a  soft  bed  ;  and  we  have  shown  above  (p.  311)  how 
valuable  is  this  joy  of  movement  to  the  body  as  a  whole. 


CHAPTER  XVIII 

THE  HYGIENE  OF  THE  NERVOUS  SYSTEM. 
REST  AND  SLEEP 

In  no  respect  do  the  conditions  of  modern  life  stand  in 
more  striking  contrast  to  the  life  of  former  times  than  in 
the  increasing  importance  of  mental  work  in  contrast  with 
muscular  work  as  a  means  of  support.  Not  only  are  there 
more  professional  men,  —  such  as  lawyers,  editors,  physi- 
cians, teachers,  and  the  like,  —  but  the  character  of  modern 
business  life  involves  no  less  the  use  of  the  nervous  system 
both  on  the  part  of  those  who  direct  large  enterprises  and 
of  those  who  occupy  subordinate  positions.  The  clerk  in 
a  bank,  as  well  as  the  president  or  cashier,  is  "living  by 
his  wits  "  and  is  using  his  brain  to  an  extent  rarely  seen 
until  within  the  last  century.  Never  was  competition  so 
keen;  never  has  it  been  so  necessary  to  inform  oneself 
minutely  as  to  market  conditions  of  demand  and  supply; 
never  before  has  the  margin  of  profits  been  so  small ;  never 
before  has  it  been  so  necessary  to  avoid,  waste ;  and  never 
before  has  it  been  so  difficult  to  protect  oneself  against 
novel  and  unforeseen  conditions.  Truly  the  modern  busi- 
ness man  must  be  ever  awake,  ever  alert. 

Nor  is  this  all.  With  the  introduction  of  the  telegraph 
and  telephone,  communication  between  man  and  man  is 
facilitated;  the  widespread  employment  of  stenographers 
results  in  an  increase  of  letters  received  and  sent;  and 
in  other  ways  the  number  of  matters  demanding  atten- 
tion is  multiplied  many-fold.  Moreover,  the  increase  of 
wealth  has  enlarged  the  possibilities  of  life  ;  concerts,  art 

334 


HYGIENE  OF  THE  NERVOUS  SYSTEM       335 

exhibitions,  books,  crowd  upon  us ;  social  engagements 
are  multiplied;  so  that  as  a  result  we  are  kept  ever  on 
the  alert,  and  the  man  or  woman  who  does  not  firmly 
decline  invitations,  engagements,  and  efforts  which  would 
overcrowd  life  to  no  good  purpose,  experiences  elements 
of  distraction,  or  fatigue,  or  worry,  which  tell  upon 
health  and  too  often  lead  to  what  we  call  nervous  pros- 
tration. 

For  no  student  of  the  practical  problems  of  hygiene 
can  shut  his  eyes  to  the  marked  prevalence  of  nervous 
prostration  and  even  insanity,  or  fail  to  recognize  the 
evident  connection  between  these  things  and  the  intensity, 
the  hurry,  the  unrestful  character  of  the  lives  we  lead. 
Probably  there  is  no  more  pressing  problem  of  practical 
hygiene  than  that  which  is  here  presented. 

And  even  where  there  is  no  question  of  nervous  prostra- 
tion or  insanity,  a  large  number  of  people  suffer  from 
nervous  troubles  of  one  kind  or  another  which  interfere 
seriously  with  their  work  and  with  the  legitimate  enjoy- 
ment of  life.  We  have  seen  how  close  is  the  connection 
between  all  parts  of  the  nervous  system,  and  also  how  condi- 
tions of  the  nervous  system  may  and  must  influence  nutri- 
tive and  other  functions  of  the  body.  The  two  are  most 
intimately  bound  together,  and  many  a  man  or  woman 
fails  to  secure  the  blessing  of  good  health  because  in- 
tense, unremitting  work  is  demanded  of  the  nervous  sys- 
tem, such  as  would  never  be  imposed  on  the  muscles,  or 
the  stomach,  or  the  skin.  Consequently  the  avoidance  of 
actual  nervous  prostration  is  but  a  small  part  of  what 
must  be  accomplished  by  the  hygienic  conduct  of  life ; 
a  far  more  pressing  practical  problem  is  the  lessening  of 
daily  strain,  worry,  and  fatigue  which  are  the  precursors 
of  the  more  serious  troubles,  and  the  avoidance  of  which 
affords  the  only  sure  means  of  defense  against  the  all  too 
common  and  distressing  breakdowns  of  useful  lives. 


336  THE  HUMAN  MECHANISM 

Christian  nations  have  always  had  in  Sunday  one  day 
in  seven  when  most  of  the  work  of  life  may  be  suspended 
and  the  strain  relieved;  and  the  preservation  as  far  as 
possible  of  Sunday  as  a  day  of  rest  is  a  matter  of  sound 
hygienic  policy.  Recent  years  have  also  shown  encour- 
aging signs  of  relief  from  steady  strain  in  shorter  hours  of 
labor,  in  the  early  closing  of  shops  on  Saturday,  and  in  the 
more  general  use  of  vacations  in  summer.  These  are  all  to 
be  welcomed  and  encouraged,  but  a  large  amount  of  strain 
still  remains;  the  value  of  the  Sunday  rest  is  largely  nullified 
when  we  go  back  on  Monday  morning  to  an  intense  appli- 
cation which  continues  without  break  until  Saturday  night. 

1.  What  causes  Nervous  Strain  ?  —  The  nervous  system, 
in  common  with  all  other  organs  of  the  body,  is  unfavor- 
ably affected  by  unwholesome  conditions  or  acts  of  life. 
Improper  feeding,  sedentary  occupations,  bad  ventilation, 
or  overheating  of  houses  tell  upon  its  working  capacity; 
and  the  effects  of  such  conditions  upon  the  nervous  sys- 
tem are  often  wrongly  attributed  to  mental  work  or  over- 
work.   Especially  is  this  true  in  the  matter  of  muscular 
activity.     Many  break  down  with  nervous  prostration,  not 
because  the  brain  and  spinal  cord  have  been  overworked, 
but  because,  in  an  excessive  devotion  to  business,  or  sci- 
ence, or  literature,  or  art,  or  pleasure,  or  even  because  of  sheer 
idleness,  the  muscular  system  has  been  neglected  and  has 
accordingly  failed  to  minister  to  the  rest  of  the  body.  At 
the  same  time  it  often  happens  that  despite  proper  mus- 
cular activity,  proper  feeding,  etc.,  the  element  of  strain 
is  still  present.    Muscular  activity  itself  entails  extensive 
nervous  work,  and  the  nervous  activity  which  participates 
in  muscular  work  may  be  only  a  part  of  the  sum  total 
which  produces   overstrain,   and   even   leads  at  times   to 
nervous  prostration  itself. 

2.  Misdirected  Nervous  Activity.  —  It  is  very  important 
to  understand  clearlv  that  it  is  misdirected  nervous  activity, 


HYGIENE  OF  THE  NEE  YOU  S  SYSTEM       337 

and  not  mental  work  in  itself,  or  the  concentration  of 
attention  which  mental  work  requires,  that  leads  to  bad 
results.  It  is  a  part  of  our  normal  life  to  do  mental  work 
and  to  cultivate  the  power  of  close  application  to  that  work ; 
it  is  a  part  of  education  to  develop  the  power  of  concentra- 
tion and  attention  against  resistance  and  inclination,  and 
experience  shows  that  this  may  be  entirely  consistent  with 
the  maintenance  of  health.  But  when  a  student  "  crams  " 
for  an  examination  for  two  or  three  days,  with  the  minimum 
of  sleep  during  the  period,  and  breaks  down  after  it  is 
over,  it  is  not  merely  mental  work  which  should  be  blamed 
for  the  result;  for  he  would  probably  have  broken  down 
if  he  had  attempted  to  work  a  typewriter  during  the  same 
time,  with  no  more  relaxation  or  rest.  The  real  cause  of  the 
trouble  is  the  too  long  continued  use  of  the  nervous  system. 

3.  Mental  Work  and  Overwork. —  Much  nonsense  is  said 
and  written  about  "  working  the  brain  too  hard."  If  by 
this  is  meant  working  it  too  long  at  a  time  without  rest, 
or  without  stopping  for  muscular  exercise ;  if  it  means  the 
attempt  to  do  more  sums  in  arithmetic,  to  read  more  Latin 
or  German,  to  write  a  longer  composition,  or  to  master 
more  science  than  the  hours  of  study  justify,  and  so  pro- 
long these  hours  of  study  to  the  neglect  of  other  hygienic 
demands,  no  objection  can  be  made  to  the  phrase;  but  if 
it  refers  to  the  hard  mental  work  and  close  application 
required  for  a  reasonable  time  by  a  sum  in  mathematics  or 
a  passage  in  Latin,  we  may  well  hesitate  to  regard  such 
work  as  in  any  degree  dangerous.  The  world  is  overflow- 
ing with  people  who  have  never  acquired  habits  of  mental 
concentration  and  hard  thinking ;  and  yet  their  general 
health  is  no  better  than  that  of  persons  who  have  acquired 
such  habits,  while  their  mental  powers  often  suffer  severely 
by  comparison. 

In  the  physiological  portion  of  this  work  the  anatom- 
ical and  physiological  aspects  of  the  nervous  system  were 


338  THE  HUMAN  MECHANISM 

carefully  described.  One  of  the  chief  reasons  for  doing  this 
was  to  impress  upon  the  student  the  extreme  complexity 
of  the  mechanism,  the  great  number  of  parts  (neurones) 
which  are  concerned  in  our  actions,  and  the  natural  diffi- 
culty, as  well  as  the  necessity,  of  proper  coordination.  In 
the  work  of  a  muscle  it  is  not  so  important  if  some  of  the 
fibers  fail  to  do  their  work,  provided  the  remaining  fibers 
work  harder,  and  so  exert  the  same  pull  on  the  tendon, 
for  the  work  will  still  be  done.  But  it  is  not  so  with  the 
nervous  system ;  if  ten  or  twenty  neurones  of  a  given 
nervous  mechanism  fail  to  work,  the  work  will  not  be 
done  at  all,  or  will  at  best  be  done  imperfectly.  In  any 
mechanism  of  interdependent  parts,  weakness  of  one  part 
means  weakness  of  the  whole.  The  secret  of  efficiency  in 
the  nervous  system  as  a  whole  is  the  maintenance  of  the 
efficiency  of  each  and  every  unit. 

4.  The  Care  of  the  Nervous  Machinery.  Rest  and  Sleep. 
-If  a  locomotive  is  to  be  kept  in  the  state  of  high  effi- 
ciency, it  must  not  be  worked  without  ceasing  until  some- 
thing goes  wrong.  When  a  train  is  to  be  pulled  three 
hundred  miles  it  is  customary  to  change  engines  two  or 
three  times  on  the  run;  and  these  changes  are  made,  not 
because  the  first  engine  cannot  pull  the  train  to  its  desti- 
nation on  schedule  time,  but  because  heating  occurs,  or 
dust  finds  its  way  into  the  bearings,  or  the  strains  and  jars 
impair  adjustment ;  and  it  prolongs  the  life  of  the  machine 
and  its  good  working  to  remove  the  dust,  cool  the  parts, 
and  otherwise  frequently  put  the  engine  in  perfect  order. 
When  an  engine  breaks  down,  it  is  usually  because  some 
one  part  has  given  way.  With  proper  care  a  good  machine 
should  wear  out  but  not  break  down. 

The  central  nervous  system,  although  infinitely  more 
complicated  than  the  steam  locomotive,  is  far  less  durable 
as  a  mechanism.  Its  bearings  are  not  made  of  hard  steel, 
but  of  living,  irritable  protoplasm  keenly  susceptible  to 


HYGIENE  OF  THE  NERVOUS  SYSTEM       339 

fatigue.  In  the  numerous  connections  between  neurone 
and  neurone  there  is  the  same  chance  as  in  the  steam 
engine  that  some  one  part  will  fail  to  do  its  work ;  and  the 
main  principle  of  its  hygienic  care  is  to  oil  the  bearings  and 
clean  and  repair  the  machinery,  by  repose  and  sleep,  before 
the  danger  of  a  breakdown  is  imminent.  Rest,  and  especially 
the  rest  of  sleep,  is  the  one  preventive  for  these  unfavor- 
able conditions ;  by  these  alone  is  the  fatigued  neurone 
withdrawn  from  work  and  given  the  chance  to  repair  itself 
and  to  return  to  its  normal  condition. 

5.  How  Much  Sleep  is  Advisable?  —  Different  people 
undoubtedly  require  different  amounts  of  sleep  ;  but  it  is 
safe  to  say  that  the  vast  majority  of  adults  require  from 
seven  to  eight  hours  a  day  ;  children  and  young  people 
require  more.  It  is,  however,  an  interesting  question 
whether  all  of  this  should  be  taken  at  one  time  or  not. 
Since  the  nervous  life  of  to-day  is  more  intense  than  was 
that  of  our  ancestors,  it  is  all  the  more  needful  that  we 
keep  the  nervous  system  in  a  continuous  state  of  high 
efficiency.  To  go  about  the  duties  and  pleasures  of  life 
from  early  morning  until  late  at  night  without  a  moment's 
rest  is  a  great  mistake  ;  we  are  then  doing  what  the 
engineer  would  do  who  should  run  his  engine  all  day, 
feeding  it  with  coal  but  without  giving  it  a  drop  of  oil, 
without  tightening  a  nut,  without  cleaning  a  bearing.  As 
the  play  of  nervous  activity  goes  on,  now  calling  upon  one 
combination  of  nerve  cells,  now  upon  another  combination, 
those  nerve  cells  which  belong  to  more  than  one  mechan- 
ism are  called  on  for  more  than  their  share  of  work,  and 
every  mechanism  to  which  they  belong  may  be  to  that 
extent  impaired.  The  stimulus  of  the  will  must  be  more 
vigorously  applied,  and  as  this  becomes  ineffective,  the 
individual  is  tempted  to  use  stimulants,  as  the  whip  is 
applied  to  tired  and  straining  horses,  or  as  blows  were 
showered  upon  galley  slaves  in  time  of  battle. 


340  THE  HUMAN  MECHANISM 

Contrast  with  this  the  benefit  of  brief  sleep  during  the 
day  in  facilitating  night  work.  Some  persons,  it  is  true, 
do  not  seem  to  be  thus  benefited,  but  the  vast  majority 
are.  And  the  benefit  is  out  of  all  proportion  to  the  time 
spent  asleep.  We  are  tired  and  work  is  difficult,  not  so 
much  because  the  whole  nervous  system  is  exhausted,  but 
because  unfavorable  conditions  of  fatigue,  etc.,  have  come 
in  at  important  points ;  during  even  a  short  nap,  with  its 
marked  muscular  and  nervous  relaxation,  normal  condi- 
tions are  restored  and  the  whole  mechanism  then  works 
on  with  less  effort,  less  general  fatigue,  less  local  injury. 

6.  Nervous  Rest  in  Change  of  Work.  —  Sleep  is  the  very 
best  means  of  insuring  local  nervous  repair,  because  it  is 
the  only  condition  which  involves  complete  relaxation. 
There  is,  however,  some  rest,  or  at  least  some  refreshment, 
in  mere  change  of  employment;  as  when,  for  instance, 
we  pass  from  mental  work  to  physical  exercise.  Calling 
into  play  a  new  group  of  nerve  cells  gives  a  chance 
for  rest  to  many  cells  which  have  previously  been  active. 
And  at  times  we  feel  tired  after  mental  work  because  we 
need  muscular  activity  rather  than  sleep.  The  tired  feel- 
ing may  come  not  from  tired  nerve  cells  but  from  the 
want  of  what  the  muscles  might  furnish  (see  Chapter 
XVII).  At  such  times  muscular  exercise  to  some  extent, 
perhaps  to  a  great  extent,  refreshes  us ;  and  in  general  we 
maintain  a  higher  degree  of  working  power  by  judicious 
variety  of  activity.  But  it  must  be  remembered  that,  in 
the  long  run,  neither  muscular  exercise  nor  any  other 
change  of  occupation  can  take  the  place  of  the  complete 
relaxation  and  refreshment  found  in  sleep.  It  is,  indeed, 
doubtful  whether  there  is  any  change  of  employment  which 
brings  with  it  an  entire  change  of  nervous  activity.  A 
certain  number  of  the  same  cells,  already  weary,  are  still 
kept  at  work,  as  has  already  been  explained  above ;  and  it 
is  by  sleep  alone  that  every  cell  has  its  natural  opportunity 


HYGIENE  OF  THE  NERVOUS  SYSTEM       341 

for  repair.  Those  who  would  define  rest  as  "selected 
excitement "  should  bear  this  fact  carefully  in  mind. 

The  cardinal  principle  in  the  care  of  the  nervous  system 
is  thus  the  same  as  that  in  the  care  of  the  steam  engine. 
Do  not  often  call  upon  it  for  activity  of  any  kind  when 
conditions  of  undue  fatigue  are  likely  to  be  present.  Go 
to  the  performance  of  every  physiological  activity,  to  diges- 
tion, to  study,  to  muscular  work,  to  social  life,  — for  all 
these  mean  nervous  activity,  —  as  far  as  may  be  with  a 
rested  nervous  system.  Of  course  to  do  this  is  not  always 
possible  ;  there  are  times  when  we  must  drive  the  body  to 
mental  work  despite  the  fact  that  it  is  physically  tired  ; 
but  this  ought  to  be  the  exception,  never  the  regular  order 
of  life. 

7.  Examples.  —  Let  us  suppose  that  some  one,  man  or 
woman,  after  application  at  sedentary  work  for  six  or 
eight  hours,  has  some  time  free  before  the  evening  meal, 
and  that,  tired  and  perhaps  nervous,  relaxation  is  sought 
in  a  brisk  walk,  which  is  almost  immediately  followed  by 
dinner.  The  effort  which  the  digestion  of  this,  perhaps 
the  heaviest  meal  of  the  day,  costs  the  nervous  system 
shows  itself  in  a  stupid,  almost  somnolent  condition  which 
often  follows.  The  body  is  trying  to  do  hard  work  with  a 
tired  nervous  system,  some  of  whose  bearings  need  oiling ; 
its  owner  is  making  the  mistake  of  continued  activity 
without  opportunity  for  the  rest  and  repair  which  a  nap 
of  fifteen  or  twenty  minutes,  or  even  absolute  idleness  and 
complete  muscular  relaxation  without  sleep,  for  half  an 
hour  or  so  before  the  meal,  might  have  given  him. 

Again,  there  are  times  in  every  one's  life  when  some 
unusual  strain  must  be  borne  ;  when,  for  example,  after 
the  day's  work  watch  must  be  kept  at  a  sick  bed  during 
the  greater  part  of  the  night.  Too  often  people  will 
undertake  this  strain,  expecting  to  "  make  up  "  the  loss  of 
rest  when  it  is  over,  even  when  it  is  possible  to  prepare 


342  THE  HUMAN  MECHANISM 

for  it  by  an  hour  or  so  of  sleep  beforehand.  We  seldom 
work  steam  engines  in  this  way.  Should  we  treat  the 
nervous  system  less  carefully  than  a  steam  engine? 

These  examples  must  suffice.  The  application  must  be 
made  by  each  individual  according  to  his  work  in  life.  If 
work  is  undertaken  which  requires  constant  activity  from 
early  morn  until  late  at  night,  the  case  is  hopeless,  and 
the  only  remedy  is  a  change  of  occupation.  Only  gross 
ignorance  of  the  plainest  facts  of  human  experience,  as 
well  as  of  physiological  science,  can  excuse  such  conduct. 

8.  Muscular  Relaxation  in  Sleep. —  Sleep  and  rest  involve 
muscular  relaxation.  All  have  noticed,  when  falling  off  to 
sleep,  the  feeling  of  relief  from  strain ;  the  framework  of 
the  skeleton  seems  to  be  held  together  less  rigidly,  and 
finally,  as  we  lose  consciousness,  relaxation  seems  com- 
plete. And  at  times  when  sleep  will  not  come,  many 
have  felt  the  inability  to  relax  ;  when,  as  it  has  been  well 
expressed,  we  seem  to  be  afraid  that  the  bed  will  slip 
away  from  under  us  and  we  must  hold  on  to  it.  We  have 
seen  that  during  waking  life  the  nervous  system  is  con- 
tinually sending  out  impulses  which  keep  the  muscles  in  a 
state  of  moderate  contraction,  and  thus  among  other  things 
liberate  heat  for  the  maintenance  of  the  body  temperature. 
Usually  this  tonic  activity  of  the  motor  neurones  must  be 
more  or  less  relaxed  before  sleep  will  come,  and  the  inabil- 
ity to  release  it  is  one  of  the  danger  signals  of  the  nervous 
system.  There  can  be  no  doubt  that  when  nervous  work 
is  pushed  too  hard  against  unfavorable  conditions,  the 
nerve  cells  develop  a  condition  of  excessive  irritability, 
so  that  they  are  discharged  by  afferent  impulses  or  other 
stimuli  which  would  ordinarily  not  affect  them  ;  and  they 
maintain  this  irritable  condition  even  in  the  presence  of 
general  bodily  fatigue.  Normal  rest  is,  of  course,  ex- 
tremely difficult  or  quite  impossible  under  these  condi- 
tions, which  for  this  reason  alone  should  be  attended  to  at 


HYGIENE  OF  THE  NEKVOUS  SYSTEM       343 

once.  The  trouble  may  be  in  some  general  or  special 
unhygienic  condition  of  life,  —  impaired  digestion,  insuffi- 
cient muscular  exercise,  the  presence  of  undue  anxiety, 
etc. ;  these  should  be  inquired  into  and  remedied  if  pres- 
ent ;  but  the  trouble  is  usually  the  result  of  pushing 
activity  of  different  kinds  for  too  long  periods  without 
cessation.  In  other  words,  we  have  lost  the  ability  to  relax 
because  we  have  not  practiced  relaxation. 

9.  Conservation  of  the  Ability  to  relax. --The  ability 
to  relax  is  something  which,  like  all  phenomena  of  nervous 
life,  depends  on  practice.  Indeed,  it  is  not  improbable  that 
it  is  something  more  than  a  mere  process  of  desisting  from 
activity,  and  that  direct  active  processes  of  inhibition  (see 
Chapter  XV)  are  concerned  in  it.  All  have  known  people 
who  can  go  to  sleep  the  instant  they  lie  down ;  and  they 
can  do  this  —  it  would  almost  seem  by  an  act  of  the  will 
—  because  they  have  long  done  it.  It  is  a  power  which 
can  indeed  be  cultivated  too  well ;  by  too  frequent  repeti- 
tion of  the  process  of  taking  a  nap,  and  by  sleeping  too 
long  at  night,  there  may  be  acquired  a  diminished  irrita- 
bility of  the  nerve  cells,  which  makes  attention  to  work  a 
very  difficult  matter,  and  long-sustained  attention  almost 
impossible.  Those  in  this  condition  may  escape  the  danger 
of  nervous  prostration,  but  they  impair  their  usefulness 
in  life. 

The  true  path,  as  in  other  matters  of  personal  hygiene, 
is  that  between  these  extremes.  When  one  rises  at  seven 
or  eight  in  the  morning,  a  short  period  of  rest  in  the  after- 
noon is  sufficient ;  the  persistent  practice  of  the  act  of 
relaxation  every  hour  or  less  is  apt  to  lead  to  loss  of  mus- 
cular tone  and  of  nervous  efficiency  in  general.  At  the 
same  time,  the  habit  of  momentary  relaxation  in  the  midst 
of  the  day's  work  is  a  valuable  aid,  partly  in  bettering 
conditions  at  the  time,  but  chiefly  in  retaining  the  power 
to  relax  when  it  is  wanted  for  longer  periods  of  rest. 


344  THE  HUMAN  MECHANISM 

10.  Drugs  are  Delusive  and  Dangerous.  —  The  physiolo- 
gist cannot  condemn  too  strongly  the  substitution  of  stim- 
ulants for  the  proper  regulation  of  work  and  rest.    The 
reader  will  see  at  once  what  this  course  of  action  may 
be  expected  to  accomplish  ;  the  stimulant  is  an  antagonist 
of  relaxation ;  the  nerve  cell  becomes  more  and  more  irri- 
table as  it  is  pushed  harder  and  harder;  finally  it  reaches 
either  the  condition  of  excessive  irritability  or  else  that  of 
being  unable  to  .work  without  the  stimulant.    It  has  adapted 
itself  to  the  presence  of  the  stimulant  in  its  environment, 
it  is  trained  to  work  under  those  conditions,  and  it  cannot 
work  without  them.    It  may  be  safely  asserted  that,  in  gen- 
eral, the  time  above  all  others  when  stimulants  should  not 
be  used  is  when  we  are  tired  out ;  to  use  stimulants  regu- 
larly, day  after  day,  in  place  of  rest  is  shown  by  experience 
to  be  one  of  the  most  dangerous  of  mistakes. 

Nor,  on  the  other  hand,  can  we  condemn  too  strongly 
the  use  of  narcotics  to  produce  sleep.  Probably  -none  of 
these  drugs  are  capable  of  producing  normal  sleep;  and 
while  in  times  of  emergency  the  physician  must  have 
recourse  to  them,  they  should  never  be  relied  upon  in 
place  of  the  hygienic  conduct  of  the  whole  life.  Many  of 
them,  and  some  of  those  in  common  use,  are  very  danger- 
ous, and  none  of  them  is  known  to  be  above  reproach. 

11.  The  Influence  of  Mental  and  Moral  States.  —  Finally, 
it  must  also  be  remembered  that  psychical  processes  exert 
a  profound  influence  upon  the  well-being  of  the  brain  and 
spinal   cord.    It  is  a  matter  of  common  experience  that 
emotions,  feelings,  moods,  etc.,  profoundly  influence  human 
conduct,   and  so  indirectly  affect  health,   especially  the 
health  of  the  nervous  system.    It  is  also  certain  that  they 
exert  a  more  direct  physiological  influence  on  the  bodily 
functions  ;   the  changes  which   emotions   produce  in  the 
heart  beat  are  good  examples  of  other  changes  which  are 
none  the  less  important  because  they  do  not  lend  themselves 


HYGIENE  OF  THE  NERVOUS  SYSTEM       345 

so  readily  to  observation.  The  bestowal  of  a  healthy  atten- 
tion upon  the  moral  aspects  of  conduct  is  a  legitimate  and 
essential  part^of  personal  hygiene  ;  and  it  is  not  too  much 
to  say  that  much  of  the  ill  health  from  which  men  and 
women  suffer  is  to  be  traced  primarily  to  the  absence  of 
sound  moral  sense  or  to  its  abnormal  or  perverted  devel- 
opment. Care  and  worry  often  cause  weariness  and  loss  of 
sleep  which  even  diversion  and  muscular  exercise  cannot 
overcome.  They  seldom  trouble  the  young,  but  as  age 
advances  they  are  sometimes  inevitable.  Efforts  should  be 
made  to  avoid  them,  as  far  as  possible,  by  a  wise  order- 
ing of  life,  by  forethought,  thrift,  economy,  sobriety,  hon- 
esty, and  the  like,  which  tend  to  "  a  light  heart "  and  "  a 
clear  conscience."  A  heavy  heart  and  a  clouded  conscience 
tend  to  unhappiness,  anxiety,  wakefulness,  and  other  phys- 
ical ills. 

12.  "  Mental "  Cures  of  Disease.  —  It  has  been  shown  that 
mental  conditions  are  far  from  *being  without  influence 
upon  the  activities  of  the  body,  even  leaving  out  of 
account  the  voluntary  muscles.  The  effect  of  emotions 
upon  the  heart  has  been  referred  to,  and  so  has  the  psychic 
secretion  of  gastric  juice.  It  is  known  that  the  movements 
of  the  alimentary  canal  are  readily  modified  by  events  in 
conscious  life.  In  the  hypnotic  state  the  effect  of  sugges- 
tion upon  functions  which  we  habitually  regard  as  invol- 
untary is  even  more  striking.  Facts  like  these  have  led 
many  to  the  rash  assumption  that  there  is  no  limit  to  the 
domination  of  the  mind  over  physiological  processes.  In 
numerous  cases  the  ascendency  which  some  have  gained 
over  certain  forms  of  disease  has  been  as  surprising  to 
others  as  it  has  been  gratifying  to  themselves.  Beyond 
question  the  righting  of  disordered  functions  and  the  sup- 
pression of  pain  have  been  frequently  attained,  and  this 
fact  makes  it  easy  to  see  why  so  great  a  following  has  been 
drawn  to  a  belief  in  the  universality  of  mental  power. 


346  THE  HUMAN  MECHANISM 

But  certain  dangers  are  always  involved  in  the  attempt 
to  overcome  disease  by  resolutely  forgetting  it  and  denying 
its  existence.  The  feeling  of  pain  may  at  times  be  ban- 
ished by  believing  that  it  does  riot  exist,  but  this  may  be 
quite  as  undesirable  as  self-inflicted  blindness  or  deafness. 
While  relief  from  pain  may  frequently  favor  recovery  by 
promoting  rest  and  nutrition,  it  may  at  other  times  sim- 
ply mean  the  loss  of  warnings  which  deserve  to  be  heard. 
Where  there  is  grave  organic  disease,  this  may  move  on 
to  a  fatal  issue  even  while  the  deluded  subject  consist- 
ently ignores  its  course.  It  is  not  wise  to  try  to  annul 
the  effects  of  a  disease  in  consciousness  when  both  cause 
and  effect  can  be  removed  by  rational  medical  treatment. 
Hypnotism  may  relieve  a  toothache,  but  it  is  not  claimed 
that  it  will  mend  a  decaying  tooth.  The  dentist's  filling, 
which  does  both,  is  the  type  of  medical  as  contrasted  with 
psychical  methods  in  dealing  with  acute  disease.  Espe- 
cially foolish  is  it  to  ignore  or  deny  the  actual  presence  of 
infectious  or  contagious  disease,  for  here  delay  menaces 
not  only  the  patient  but  those  about  him.  The  conse- 
quences of  this  folly,  when  confined  to  its  deluded  victim, 
may  end  in  virtual  suicide ;  when  they  extend  to  others, 
they  may  fall  little  short  of  manslaughter. 


CHAPTER  XIX 
THE  HYGIENE  OF  FEEDING 

The  present  chapter  deals  with  certain  hygienic  con- 
siderations connected  with  the  taking  of  food  into  the 
body,  —  its  preparation,  its  cooking,  its  quantity,  the  fre- 
quency of  our  meals,  and  the  adjustment  of  our  habits  of 
feeding  to  the  other  work  of  life.1 

Mankind  as  a  whole  was  probably  never  better  fed  than 
it  is  at  present.  The  opening  up  of  the  New  World  with 
its  vast  fields  of  corn  and  wheat  and  its  enormous  pas- 
tures ;  the  introduction  of  improved  methods  of  agriculture, 
agricultural  machinery,  and  education  in  agriculture ;  and 
especially  the  improvements  in  transportation  facilities 
and  in  arts  of  food  preserving  (such  as  refrigeration  and 
canning),  —  all  these  have  immensely  increased  the  avail- 
able food  supply  of  the  world  and  made  famine  and  star- 
vation much  more  rare  than  formerly.  It  is  now  only  in 
inaccessible  places,  such  as  the  central  parts  of  India,  that 
great  famines  still  occur. 

And  yet  in  the  midst  of  abundance  it  is  still  true  that 
many  men  and  women  are  poorly  nourished ;  for  it  is  the 
absorption  of  food  by  the  blood  and  not  merely  the  eating 
of  meals  which  supplies  the  needs  of  the  tissues.  Hence 
the  problem  of  alimentation  in  its  widest  sense  involves 
not  only  the  growing  of  food  on  farms  or  in  gardens,  and 

1  Many  practical  points  connected  with  alimentation  have  already  been 
considered  in  Part  I  (see  chapters  on  digestion  and  nutrition).  Special 
reference  may  be  made  to  Chapter  XIII,  p.  233,  on  the  Choice  of  Foods 
and  Nutrients. 

347 


348  THE  HUMAN  MECHANISM 

the  preservation  of  this  food  so  that  it  may  be  delivered  in 
proper  form  to  the  consumer,  but  also  the  eating  of  it  in 
such  form  and  quantities  and  at  such  times  as  will  insure 
its  proper  utilization,  by  the  processes  of  digestion,  for  the 
needs  of  the  body. 

1.  Appetite  as  a  Guide  in  Feeding.  —  Nature  herself  has 
provided  us  with  guides  in  the  choice  of  food,  and  these 
guides  are  the  sensations  of  hunger  and  thirst,  and  what 
we  sum  up  in  general  under  the  term  "  appetite."  So  long 
as  these  remain  normal  and  unperverted,  they  are  to  be 
largely  trusted ;  and,  like  all  physiological  functions,  they 
are  kept  normal  and  unperverted,  in  the  first  place,  by 
attention  to  the  general  health  of  the  entire  body.  Appe- 
tite is  apt  to  fail  or  become  untrustworthy  in  the  case  of 
men  or  women  who  are  suffering  from  lack  of  muscular 
activity  or  from  mental  worry.  The  care  of  the  appetite 
is  never  a  matter  of  direct  attention  to  the  appetite  itself, 
but  of  maintaining  the  bodily  conditions  in  which  it  nor- 
mally acts.  Consequently  the  basic  principle  in  securing 
proper  nutrition  is  attention  to  the  general  health.  A  pa- 
tient suffering  from  indigestion  once  consulted  a  wise 
old  doctor  and  began  recounting  the  foods  that  agreed  or 
disagreed  with  him,  together  with  his  innumerable  symp- 
toms, until  the  doctor  interrupted  him  by  saying,  "  The 
first  thing  you  must  do  is  to  forget  'that  you  have  a 
stomach."  The  present  chapter  is  not  written  for  people 
like  this  patient,  or  for  invalids,  or  for  others  suffering 
from  indigestion  in  any  one  of  its  thousand  forms.  It  is 
written  for  those  who  can,  and  will,  first  of  all,  take  the 
needful  muscular  exercise  and  the  needful  rest;  who  will 
pay  proper  attention  to  clothing  and  bathing,  to  the  heat- 
ing and  ventilation  of  the  home,  to  the  avoidance  of 
dampness  and  other  unfavorable  conditions  ;  who  will  not 
abuse  themselves  by  stimulants  and  narcotics.  Those  who 
prefer  not  to  belong  to  this  class,  or  who  because  of  some 


HYGIENE  OF  FEEDING  349 

constitutional  disease  cannot,  must  seek  and  depend  upon 
medical  advice  as  regards  their  habits  of  feeding. 

At  the  same  time,  to  insure  proper  digestion  and  nutri- 
tion, more  is  required  than  attention  to  general  hygiene. 
What  additional  precautions  are  required  in  the  taking  of 
food  by  persons  leading  an  otherwise  healthy  life  ?  It  is 
in  answer  to  this  question  that  we  shall  attempt  to  give 
some  suggestions. 

2.  Good  Cooking  as  an  Aid  in  Nutrition.  — It  has  already 
been  pointed  out  that  digestion  begins  with  the  prepara- 
tion of  the  food  by  cooking,  which  serves  three  purposes. 

1.  It  destroys  parasites  and  disease  germs.    The  impor- 
tance of  this   will   be   shown  and  emphasized  elsewhere 
(Chapter  XXXII). 

2.  It  renders  the  food  more  appetizing  (see  p.  113). 

3.  It  makes  some  foods  more  digestible  by  making  them 
accessible  to  the  action  of  the  digestive  juices ;  thus  the 
connective  tissue  of  animal  foods,  when  heated  in  the  pres- 
ence of  water,  swells,  and  is  more  easily  acted  on  by  the 
gastric  juice,   so  that  tough  meat  in  this  way  is   often 
made  tender  by  boiling.    The  cellulose  walls  of  the  vege- 
table foods,  on  the  other  hand,  are  softened  by  cooking, 
the  starch  granules  are  swollen,  and  their  envelopes  burst 
(see  p.  94). 

At  the  same  time  it  is  possible  to  render  food  less  diges- 
tible by  improper  cooking.  A  piece  of  meat  may  "have 
the  life  cooked  out  of  it " ;  and  egg  albumen,  which  in  the 
raw  state  mixes  rather  easily  with  the  gastric  juice,  may 
sometimes  be  boiled  to  a  leathery  consistency  which  ren- 
ders the  action  of  the  digestive  juices  a  slow  process. 

3.  Chewing  of  Food  an  Aid  to  Digestion.  —  It  is  unneces- 
sary to  dwell  at  length  upon  the  importance  of  chewing,  or 
mastication.  We  have  already  seen  that  the  word  "  diges- 
tion "  is  derived  from  the  Latin  words  dis  and  gero,  to  tear 
apart  or  separate ;  and  our  studies  of  physiology  have 


350  THE  HUMAN  MECHANISM 

shown  how  the  division  of  food  into  smaller  and  smaller 
masses  is  prerequisite  to  reasonable  rapidity  of  solution 
and  absorption.  The  student  is  also  reminded  of  what  has 
been  said  (p.  100)  concerning  the  importance  of  caring  for 
the  teeth. 

Vegetable  foods  especially  should  be  well  chewed,  partly 
because  the  cellulose  which  holds  them  together  is  not 
readily  acted  on  by  the  gastric  juice,  and  partly  because 
the  thorough  mixture  with  the  saliva  facilitates  the  gastric 
digestion  of  starch  (p.  110).  Meats  also  should  be  well 
masticated.  The  fact  that  a  dog  bolts  his  food  with  im- 
punity is  no  guide  for  civilized  man,  since,  for  one  rea- 
son, human  gastric  juice  contains  much  less  acid  and  so 
acts  less  readily  upon  connective-tissue  elements.  It  is  true 
that  the  "  quick  lunch"  thrives  in  busy  places,  but  no  one 
considers  it  hygienic. 

4.  Feeding  in  Relation  to  Gastric  Digestion.  —  In  order 
that  gastric  digestion  may  be  efficient  it  is,  of  course,  neces- 
sary that  gastric  juice  shall  be  secreted  in  proper  amount, 
and  we  have  learned  that  the  first  step  toward  this  secre- 
tion consists  in  the  pleasurable  sensations  connected  with 
the  satisfaction  of  appetite.  Consequently  it  is  one  of  the 
first  hygienic  requisites  of  gastric  digestion  that  the  food 
shall  be  appetizing,  and  that  the  condition  of  the  body  and 
especially  of  the  digestive  system  shall  be  such  that  the 
food  shall  be  eaten  with  relish.  This  is  not  the  same  thing 
as  saying  that  food  which  is  appetizing  will  be  digested; 
it  merely  means  that  food  is  more  digestible  for  being 
appetizing,  and  that,  when  it  is  not  enjoyed,  its  stay  in 
the  stomach  is  apt  to  be  unduly  prolonged.  For  this  and 
other  reasons  the  appetite  should  not  be  impaired  by  eat- 
ing candy,  or  by  visiting  the  pantry  between  meals  for 
something  to  eat ;  on  the  other  hand,  a  good  appetite 
should  be  encouraged  by  healthy  living,  by  proper  prepa- 
ration of  the  food,  and  even,  as  far  as  possible,  by  agreeable 


HYGIENE  OF  FEEDING  351 

table  appointments.  There  was  wisdom  as  well  as  pleas- 
ure in  the  old  custom  of  having  a  jester  at  the  dinner  table, 
and  there  is  reason  in  the  saying,  "Laugh  and  grow  fat." 

5.  Excessive  Quantity  of  Food.  Overfeeding.  —  It  is 
furthermore  important  that  the  amount  of  food  eaten  at 
one  time  be  not  excessive,  and  that  the  stomach  under  no 
circumstances  be  unduly  distended.  A  large  proportion  of 
those  cases  of  dyspepsia  which  have  their  origin  partly  or 
entirely  in  the  conditions  of  feeding  are  due  to  overeating, 
which  may  take  various  forms.  Too  large  a  proportion  of 
the  total  food  may  be  taken  at  one  meal,  usually  dinner  ; 
or  too  many  meals  may  be  taken,  —  three  should  suffice ; 
or  each  of  the  three  may  be  full-sized  meals,  —  a  very  unde- 
sirable custom  among  those  engaged  in  sedentary  pursuits. 
We  have  seen  that  the  one  condition  of  life  which  calls 
for  heavy  feeding  is  that  of  muscular  activity,  whether  in  the 
performance  of  external  work  or  for  the  production  of  heat 
in  cold  weather  ;  a  person  who  is  engaged  in  some  occu- 
pation which  involves  large  amounts  of  muscular  work 
can  and  should  have  three  full  meals  daily  ;  with  others 
the  habit  is  attended  with  considerable  risk. 

Gluttony  has  always  been  a  vice  of  the  idle  and  luxu- 
rious. As  the  world  has  grown  wiser  it  has  become  less 
common,  because  a  larger  intelligence  makes  it  plain  that 
gluttony  defeats  its  own  ends,  and  that  the  secret  of  the 
greatest  pleasure  in  eating,  as  in  everything,  lies  in  temper- 
ance, not  in  excess. 

Many  persons,  however,  without  any  desire  or  even  any 
thought  of  gluttony,  regularly  overeat.  These  are  usually 
healthy  persons  leading  sedentary  lives,  "  blessed,"  as  they 
say,  "  with  a  good  appetite,"  and  because  of  quiet  or  even 
indolent  disposition  giving  but  small  heed  to  muscular 
activity.  As  the  years  go  by,  such  persons  are  apt  to  grow 
fat,  and  by  and  by  to  find  themselves  suffering  from  a  weak 
heart,  or  shortness  of  breath,  or  something  worse ;  seldom 


352  THE  HUMAN  MECHANISM 

realizing,  until  it  is  too  late,  that  overeating  is  the  princi- 
pal cause  of  their  undoing.  If  sufficient  manual  labor  or 
other  exercise  of  the  skeletal  muscles  is  practiced,  trouble 
from  overeating  rarely  comes.  It  is  the  sedentary,  inactive, 
and  indolent  who  suffer  most  from  this  source ;  for  them 
a  good  appetite  often  proves  to  be  a  curse  rather  than  a 
blessing,  and  a  poor  appetite,  by  preventing  overeating, 
has  often  been  a  blessing,  though  a  blessing  in  disguise. 

6.  Fried  Foods Caution  is  required  in  the  use  of  fried 

foods.    When  a  layer  of  fat  varnishes  over  a  particle  of 
food  the  digestive  juices  do  not  readily  penetrate  the  mass, 
and  digestion  is  to  that  extent  impaired.    This  is  not  of 
so  much  importance  in  intestinal  digestion,  since  in  that 
portion  of  the  alimentary  canal  the  layer  of  fat  is  itself 
digested  and  removed  ;  the  stomach,  on  the  other  hand, 
does  not  digest  fat,  and  we  can  easily  see  how,  because  of 
its  interference  with  the  first  processes  of  digestion  in  this 
organ,  the  use  of  too  much  fried  food  is  unwise. 

Moreover,  in  frying,  care  should  be  taken  to  have  the 
temperature  of  the  fat  high  enough  to  coagulate  promptly 
the  surface  layers  of  the  food,  thus  preventing  the  pene- 
tration of  the  fat  into  the  food,  which,  moreover,  should 
not  be  served  swimming  in  fat,  but  as  dry  as  possible. 
The  frying  pan  is  still  used  far  too  extensively  in  some 
parts  of  America.  Most  of  our  foods  should  be  roasted, 
broiled,  boiled,  or  baked,  rather  than  fried. 

7.  Perspiration  in  Relation  to  the  Hygiene  of  Feeding.  - 
The   secretion  of  gastric  juice   is  seriously  impaired  by 
excessive  perspiration,   especially   when  the  loss   to   the 
system  is  not  made  good  by  drinking  sufficient  amounts 
of  water.    This  is  probably  true  of  the  secretion  of  all  of 
the  digestive  juices,  but  it  is  especially  important  in  the 
case  of  the  gastric  digestion,  upon  the  proper  performance 
of  which  the  subsequent  work  of  intestinal  digestion  so 
largely   depends.     Therefore,    in   general,    smaller    meals 


HYGIENE  OF  FEEDING  353 

should  be  eaten  in  hot  weather,  —  we  have  seen  that  we 
need  less  food  at  that  time,  —  and  heavy  meals  should  not 
be  taken  immediately  after  vigorous  exercise  involving 
profuse  perspiration.  Indeed,  it  is  a  general  rule  that  ex- 
cessive loss  of  water  by  perspiration  should  be  made  good, 
as  far  as  possible,  by  drinking  water  more  freely. 

8.  Digestion  and  Bodily  Fatigue. — Digestion,  like  all 
other  functions  of  the  body,  involves  to  a  very  considerable 
extent  the  intervention  of  the  nervous  system ;  and  we 
may  repeat  here  the  advice  already  given  (p.  341)  not  to 
go  tired  to  the  digestion  of  a  heavy  meal.    It  is  one  of  the 
objections,  probably  the  chief  objection,  to  evening  dinners 
that  they  so  frequently  follow  immediately  upon  a  hard 
day's  work,  when  the  nervous  system  is  in  a  poor  condi- 
tion for  its  share  in  digestive  work.    A  rest  of  half  an 
hour  before  dinner  is,  however,  generally  all  that  is  needed, 
and  usually  prevents  the  mental  heaviness  which  so  often 
follows  a  full  meal. 

9.  Mental  Work  after  Meals.  —  An  exaggerated  impor- 
tance has  probably  been  given  at  times  to  the  danger  of 
mental  work  after  meals.    There  is  no  proof  whatever  that 
the  demand  of  the  brain  for  greater  blood  supply  will 
seriously   interfere    with    that   to    the   digestive    organs. 
While  it  is  true  that  indigestion  often  affects  people  who 
go  straight  from  their  meals  to  hard  mental  work,  it  is 
also  true  that  these  are  usually  people  who  take  insuffi- 
cient muscular  exercise,  rest,  and  sleep.    The  relation  of 
the  circulation  in  the  brain  to  that  in  the  digestive  organs 
is  too  imperfectly  understood  to  justify  some  of  the  glib 
but  shallow  utterances  frequently  met  with  on  this  subject, 
especially  when  the  statements  in  question  are  not  clearly 
supported  by  experience.    (See  page  155.) 

10.  Muscular  Activity  after  Meals — Vigorous  muscular 
activity  immediately  after  meals  is  quite  another  matter. 
Here  we  know  that  blood  is  taken  away  from  the  digestive. 


354  THE  HUMAN  MECHANISM 

organs  and  sent  through  the  muscles  and  skin;  this  fact 
suggests  caution,  and  experience  amply  confirms  the  need 
of  the  caution  thus  suggested.  Even  here  it  is  vigorous 
exercise,  and  especially  after  heavy  meals,  that  is  to  be 
condemned. 

11.  The  Use  of  Water  as  a  Drink.  —  Many  people,  and 
especially  many  women,  drink  too  little  water.  Water  is 
constantly  being  lost  through  the  lungs,  skin,  or  kidneys, 
and  this  loss  is  only  partially  made  good  by  the  oxidation  of 
the  hydrogen  of  the  proteids  and  fats.1  No  rules  as  to  the 
amount  can  be  given,  since  it  varies  so  much  with  tem- 
perature and  the  amount  of  muscular  activity  ;  but  the 
habit  of  drinking  no  water  between  meals  and  but  little  at 
the  table,  in  spite  of  popular  opinion  on  the  subject,  is  to 
be  strongly  deprecated.  We  have  already  shown  that  the 
abstraction  of  undue  amounts  of  water  by  perspiration 
may  seriously  interfere  with  the  secretion  of  the  gastric 
juice,  and  there  is  every  reason  to  believe  that  a  deficiency 
in  the  supply  of  water  to  the  blood  similarly  interferes 
with  the  secretion  of  the  other  digestive  juices,  and  so,  by 
impairing  intestinal  digestion,  favors  constipation. 

Undue  emphasis  has  been  laid  upon  the  danger  of  drink- 
ing water  with  meals.  The  reasons  given  —  that  such 
water  unduly  dilutes  the  gastric  juice  or  takes  the  place 
of  a  normal  secretion  of  saliva  —  are  questionable.  As  a 
matter  of  fact,  the  water  thus  taken  is  soon  discharged  into 
the  intestine  and  absorbed.  It  is  true,  however,  that  the 
use  of  too  much  fluid  with  the  meal  is  apt  to  lead  to 
insufficient  mastication  because  it  makes  it  easier  to  swal- 
low the  food  ;  and  from  this  point  of  view  caution  is 
advisable.  It  is  probably  also  true  that  much  drinking 
with  meals  tends  to  overeating,  by  facilitating  rapid  eating ; 

1  The  water  excreted  from  the  body  comes  partly  from  the  water  drunk, 
but  also  partly  from  that  formed  by  the  union  of  the  hydrogen  of  the 
food  with  oxygen. 


J 


HYGIENE  OF  FEEDING  355 


and  it  may  be  that  this  is  one  reason  why  fat  people  are 
usually  great  drinkers. 

A  further  point  in  the  hygiene  of  gastric  and  intestinal 
digestion  is  the  avoidance  of  those  inflammatory  conditions 
of  the  bowels  which  follow  exposure  to  cold.  This  sub- 
ject will  be  dealt  with  in  Chapter  XXI.  The  student 
will  also  recall  what  has  been  said  in  Chapter  XVII  with 
regard  to  the  importance  of  general  muscular  exercise,  and 
especially  of  exercises  involving  the  use  of  abdominal 
muscles. 

12.  The  Importance  of  Coarse  Foods.  —  In  treating  of 
the  physiology  of  digestion  it  was  pointed  out  that  the 
presence  of  a  certain  amount  of  indigestible  material  in 
the  food  is  helpful  as  a  stimulus  to  the  muscular  action  of 
the  intestine.    Food  may  be,  and  nowadays  often  is,  espe- 
cially in  "  delicate  dining,"  too  largely  composed  of  very 
digestible  substances,  which  leave  an  insufficient  residue 
of  solid  material  in  the  lower  intestine  to  stimulate  proper 
peristalsis.    Our  main  reliance  for  the  needed  coarse  or  in- 
digestible part  of  our  diet  is  the  cellulose  of  many  of  our 
vegetable  foods ;  oatmeal  and  fruit  are  two  of  the  more 
important  foods  which  serve  the  purpose.    Indian  corn  is 
also  an  important  laxative,  although  its  action  is  probably 
dependent  on  other  things  than  cellulose.    The  laxative 
action  of  prunes  and  figs  is  well  known.    Graham  flour, 
bran,  "whole  wheat,"  etc.,  which  retain  more  or  less  of  the 
hull  of  the  grain,  have  the  same  action.    In  selecting  the 
diet  care  should  be  taken,  especially  in  winter,  to  include 
in  it  a  sufficient  quantity  of  such  foods,  and  a  judicious 
addition  of  coarse  foods  to  any  diet  is  probably  wise. 

13.  The  Individual  must  study  his  own  Needs. —  In  thus 
sketching  the  broad  outline  of  hygienic  feeding  little  or 
no  attention  has  been  given  to  what  we  should  or  should 
not  eat;  and  this  has  been  done  in  order  to  discourage 
looking  at  the  subject    from   this   popular   but   entirely 


356  THE  HUMAN  MECHANISM 

misleading  point  of  view.  It  may  be  true  that  "what  is  one 
man's  meat  is  sometimes  another  man's  poison,"  but  only 
in  a  very  limited  sense.  Each  individual  in  the  course  of 
his  experience  will  learn  that  there  are  some  things  he 
cannot  eat  with  impunity,  and,  if  he  be  wise,  will  gov- 
ern himself  accordingly.  But  it  must  be  remembered  that 
man  enjoys  a  wide  latitude  in  the  choice  of  his  food.  The 
vast  majority  of  people,  if  they  will  but  lead  otherwise 
hygienic  lives,  can  eat  almost  anything ;  and  the  inability 
to  digest  something  which  we  have  always  eaten,  or  which 
others  eat  with  impunity,  should  lead  not  so  much  to 
its  exclusion  from  the  diet  as  to  an  inquiry  whether  the 
trouble  does  not  have  its  origin  in  the  general  unhygienic 
conduct  of  life.  Those  who  treat  such  conditions  by  con- 
structing a  table  of  the  things  they  can  eat  and  another 
of  those  they  cannot  eat,  and  confine  their  diet  to  the 
former,  usually  find  that  as  life  advances  the  size  of  the 
latter  table  increases  at  the  expense  of  the  former.  It  is 
the  same  fallacy  of  dealing  with  the  symptom  instead  of 
the  disease,  which  leads  others  to  treat  constipation  with 
cathartics,  and  still  others  to'  treat  a  bronchial  cough  with 
so-called  "  cough  medicines." 


CHAPTER  XX 
FOOD  ACCESSORIES,  DRUGS,  ALCOHOL,  AND  TOBACCO 

1.  Food  Accessories  and  Drugs. — Through  the  alimentary 
and  respiratory  tracts  there  are  received  into  the  blood  not 
only  substances  such  as  proteids,  gelatin,  fats,  carbohy- 
drates, salts,  and  water,  which  we  have  described  as  sup- 
plying the  material  for  power  and  for  growth  and  repair, 
but  also  other  substances  capable  of  modifying  in  one  way 
or  another  the  course  of  events  within  the  body.  The 
flavors  which  contribute  to  the  enjoyment  of  foods  play 
an  important  r61e  in  the  secretion  of  the  gastric  juice  ; 
and  yet  the  substances  which  cause  these  flavors  are  negli- 
gible as  sources  of  power.  Salt  belongs  under  the  same 
head;  for  we  use  in  cooking  more  salt  than  is  needed 
to  make  good  the  daily  loss  from  the  body,  and  we  do  this 
to  develop  an  agreeable  flavor  in  our  food.  Substances 
of  this  kind  are  spoken  of  as  food  accessories,  and  among 
them  must  be  included  coffee  and  tea,  for  their  effect  is 
not  chiefly  a  matter  of  nutrition ;  certain  constituents  of 
tea  and  coffee  absorbed  into  the  blood  affect  the  ner- 
vous system,  and  it  is  largely  for  this  reason  that  we  use 
them. 

We  may  pass  in  this  way  from  the  necessary  food  acces- 
sories through  those,  like  coffee  and  tea,  which,  while 
not  essential,  may  still  be  regarded  as  part  of  the  food  of 
a  large  portion  of  mankind,  to  the  great  number  of  chem- 
ical compounds  known  as  drugs,  which  also  act  by  chan- 
ging the  course  of  events  within  the  body ;  and  it  is  difficult 
to  draw  any  sharp  line  of  distinction  between  those  which 

357 


358  THE  HUMAN  MECHANISM 

occasionally  serve  as  medicine  or  "stimulants"  and  those  of 
which  daily  use  is  made  as  food  accessories. 

Animals  as  a  rule  take  substances  into  their  bodies  only 
to  satisfy  hunger  or  thirst  or  appetite  ;  man  alone  takes, 
in  addition  to  his  nutriment,  food  accessories  and  drugs 
for  the  sake  of  their  special  effect  upon  the  nervous  sys- 
tem or  other  organs.  Many  of  the  numerous  food  acces- 
sories which  human  ingenuity  has  discovered  or  devised 
are  harmless  enough  in  the  form  used ;  but  others  con- 
tain substances  which  are  capable  of  poisoning  the  body. 
It  is  'an  important  part  of  the  study  of  personal  hygiene  to 
learn  of  what  these  substances  consist,  what  is  their  ac- 
tion on  the  human  organism,  and  wherein  lie  their  special 
dangers. 

2.  The  Drug  Habit.  —  It  is  a  lamentable  fact  that  large 
amounts  of  drugs  are  swallowed  by  men  and  women  apart 
from  any  medical  need  which  compels  their  use.    In  a  sub- 
sequent chapter  we   shall  show  reasons  for  avoiding  an 
undue   dependence  upon  drugs  as  a  remedy  for  various 
minor  ills.     Bad  as  this  practice  is,  with  its  tendency  to 
rely  upon  the  uncertain  action  of  a  drug  instead  of  taking 
proper  hygienic  care  of  the  body,  it  is  far  worse  to  make 
habitual  use  of  drugs  for  their  special  effects  upon  the 
healthy  body,  for  the  habit  is  one  which  is  only  too  easily 
cultivated.    There   is   no    reason   why   a    healthy  human 
being,  living  a  normal  life  amid  healthful  surroundings, 
should  need  to  use  drugs  habitually,  and  a  little  consider- 
ation will  show  that  the  practice  is  dangerous. 

3.  Dangers  of  the  Drug  Habit.  —  When  we  eat  meat,  or 
vegetables,  or  when  we  breathe  air,  we  take  into  the  body 
materials   needed  for  normal  living.    These  things  have 
always  formed  part  of  the  food  of  the  race,  and,  unless 
wrongly  taken,  do  good  and  not  harm.    When,  on  the  other 
hand,  we  take  a  drug,  such  as  chloroform,  or  cocaine,  or 
opium,  or  alcohol,  or  coffee,  or  tea,  we  take  something 


DRUGS,  ALCOHOL,  AND  TOBACCO  359 

which  is  foreign  to  the  body,  in  so  far  as  it  has  not  been  a 
regular  constituent  of  animal  food  in  the  past.  It  is  not 
needed,  as  proteid  and  salt  and  water  are  needed  ;  there 
is  no  special  preparation  for  its  reception;  and,  while  it 
may  do  good,  there  is  danger  that  it  may  do  harm. 

In  the  second  place,  the  exact  action  of  many  drugs  is 
only  imperfectly  understood.  In  an  emergency  the  physi- 
cian uses  them  temporarily,  for  some  effect  which  he  desires 
to  produce,  thus  tiding  over  a  difficulty.  He  uses  the  drug 
only  a  few  times  at  most,  and  is  consequently  not  greatly 
concerned  about  unfavorable  attendant  effects  ;  it  accom- 
plishes some  needed  purpose,  and  if  it  does  any  harm,  the 
organism  may  be  trusted  to  recover  from  it.  It  is  very 
different,  however,  with  the  habitual  use  of  any  drug. 
The  very  fact  that  it  gives  some  new  direction  to  the 
events  taking  place  within  the  body  means  that  abnormal 
conditions  of  life  are  being  maintained;  and  we  have 
already  learned  that  abnormal  conditions  of  life  are  apt  to 
be  unhygienic. 

Again,  the  use  of  drugs  is  only  too  apt  to  be  substituted 
for  the  hygienic  conduct  of  life.  We  may,  for  example, 
take  drugs  to  accomplish  something  which  the  healthy 
body  should  accomplish  for  itself  without  outside  help. 
When  any  one  drinks  a  cup  of  black  coffee  to  facili- 
tate mental  work  which  his  fatigued  condition  would  not 
otherwise  allow  him  to  do,  he  is  trying  to  get  from  a  drug 
the  power  which  he  could  and  probably  should  secure 
by  normal  sleep.  The  coffee  acts  like  a  whip  to  a  tired 
horse ;  the  same  work  is  done  as  might  have  been  done 
had  the  horse  been  allowed  a  little  rest ;  but  the  horse 
is  not  as  well  off  when  he  does  the  work  under  the  lash 
as  when  he  does  it  in  a  properly  rested  condition.  Similarly, 
persons  suffering  from  sleeplessness  often  take  drugs  used 
to  produce  sleep  (hypnotics),  and,  superficially  at  least,  the 
sleep  thus  secured  resembles  normal  sleep  ;  but  experience 


360  THE  HUMAN  MECHANISM 

shows  that  few  if  any  hypnotics  can  be  used  for  any  length 
of  time  without  bad  effects.  Here  again  a  drug  is  being 
depended  upon  to  do  what  the  normal  body  should  do  for 
itself.  Pepsin  tablets  may  be  taken  to  aid  digestion,  and 
thereby  an  attack  of  indigestion  may  sometimes  be  pre- 
vented or  relieved ;  but  a  healthy  stomach  should  furnish 
its  own  pepsin  ;  and  the  fact  that  it  does  not  do  so  is  a  sure 
warning  that  something  is  wrong  in  the  conduct  of  life. 
It  is  irrational  to  neglect  the  duty  of  attending  to  the 
cause  of  the  ailment,  and  it  is  foolish  to  substitute  tem- 
porary relief  for  permanent  cure.  Perhaps  if  the  drug  did 
all  that  the  proper  care  of  the  body  does,  and  did  no  more, 
no  serious  objection  could  be  made  to  its  use  ;  but  there  is 
probably  no  drug  of  which  this  is  true,  and  for  this  reason 
it  is  foolish  and  rash  to  try  to  substitute  the  use  of  drugs 
for  the  hygienic  conduct  of  life. 

Lastly,  if  the  drugs  do  not  accomplish  in  the  long  run 
what  should  be  done  by  the  hygienic  conduct  of  life,  their 
extensive  use  becomes  all  the  more  dangerous  in  view  of 
the  unquestioned  fact  that  we  are  apt  thereby  to  become 
their  slaves.  Every  man  is  the  slave,  broadly  speaking,  of 
the  habits  he  forms,  and  it  is  only  a  question  as  to  whether 
he  will  be  the  willing  slave  of  good  habits  or  the  abject 
slave  of  bad  habits.  The  man  who  leads  a  hygienic  life  is 
the  slave  of  muscular  activity,  of  correct  feeding,  of  proper 
clothing,  of  rest,  etc. ;  that  is  to  say,  these  things  become 
necessary  to  his  life  ;  he  cannot  get  along  without  them. 
If  for  these  proper  agents  of  health  he  persistently  sub- 
stitutes some  drug,  whether  it  be  alcohol,  or  tobacco,  or 
coffee,  or  tea,  or  chocolate,  or  opium,  the  habit  of  using 
the  drug  is  substituted  for  that  of  maintaining  normal 
conditions.  But  since  drugs  cannot  entirely  take  the  place 
of  such  conditions,  the  constitution  goes  from  bad  to  worse, 
and  increasing  dependence  must  be  placed  upon  the  drug. 
It  is  a  safe  rule  that  whenever  we  are  uncomfortable  or 


DRUGS,  ALCOHOL,  AND  TOBACCO  361 

unhappy  without  the  use  of  a  certain  drug  we  should  cease 
using  it  until,  with  the  help  of  hygienic  living,  we  can  get 
along  without  it. 

There  are  people  who  are  slaves  of  coffee,  of  tea,  of 
chocolate,  of  patent  medicines,  of  candy,  and  of  soda  water, 
just  as  truly  as  there  are  slaves  of  tobacco,  or  of  alcohol, 
or  of  opium.  It  is  worse  to  be  the  slave  of  alcohol  than  of 
coffee,  because  the  evil  consequences  of  alcohol  are  greater 
than  those  produced  by  the  corresponding  use  of  coffee; 
but  it  is  by  the  same  process  in  both  cases  that  the  man 
or  woman  becomes  a  slave  to  the  drug,  and  that  process  is 
the  formation  of  bad  habits. 

With  these  practical  considerations  about  the  use  of 
drugs,  —  by  which  term  it  will  be  seen  that  we  mean,  not 
simply  the  medicines  purchased  from  the  apothecary,  but 
all  those  substances  which  are  taken  into  the  body  in  order 
to  give  some  new  or  abnormal  direction  to  the  course  of 
events  in  the  organism,  —  we  may  pass  on  to  the  discussion 
of  those  in  common  use. 

4.  Tea  and  Coffee.  —  Different  as  are  these  drinks  in 
taste  and  appearance,  their  most  important  physiological 
effects  are  due  essentially  to  the  same  substances,  viz. 
caffeine  (or  theine)  and  tannic  acid  (or  tannin).  Caffeine 
is  a  veiy  powerful  stimulant,  especially  of  the  nervous 
system,  and  also  of  the  heart,  although  probably  to  a  lesser 
degree ;  tannin,  on  the  other  hand,  is  a  bitter,  astringent 
substance,  which  may  considerably  hinder  digestion  and 
directly  injure  the  mucous  membrane  of  the  stomach.  Tea 
contains  about  twice  as  much  tannin  as  an  equal  weight  of 
coffee,  but  as  coffee  is  frequently  made  much  stronger  than 
tea  the  actual  amount  per  cup  may  often  be  more  nearly 
equal  in  the  two  drinks  than  these  figures  indicate.  The 
amount  of  tannin  dissolved  in  tea  varies  greatly  with 
the  method  of  preparation,  and  largely  for  this  reason  tea 
should  not  be  boiled,  nor  allowed  to  steep  too  long.  The 


362  THE  HUMAN  MECHANISM 

proper  method  of  making  tea  is  to  pour  over  the  dry  leaves 
water  which  has  been  brought  just  to  the  boiling  point, 
and  then  to  allow  the  infusion  to  stand,  without  further 
heating,  for  not  more  than  a  few  minutes. 

Both  tea  and  coffee  seem  to  have  a  slightly  retarding 
influence  upon  gastric  digestion.  In  healthy  people  this 
is  of  little  consequence,  but  when  the  digestive  powers  are 
in  any  way  impaired  the  use  of  these  beverages  may  be 
inadvisable.  The  more  important  effect,  however,  of  both 
tea  and  coffee  is  in  their  stimulating  action  on  the  nervous 
system.  No  satisfactory  explanation  has  yet  been  given 
of  the  fact  that  some  people  can  use  tea  and  not  coffee, 
while  with  others  the  reverse  is  true.  It  is  probably  safe 
to  say  that  when  used  in  moderation,  tea  and  coffee  are  usu- 
ally harmless  to  those  leading  an  otherwise  hygienic  life. 
They  should  be  used  sparingly  by  nervous  people  and  by 
those  in  whom  digestion  is  feeble  and  slow  (Hutchinson). 
Even  by  the  perfectly  healthy  they  should  not  be  used 
to  excess,  nor  should  the  habit  be  acquired  of  using  them 
as  the  whip  to  the  tired  horse.  Drinking  strong  coffee  in 
order  to  keep  awake  for  evening  study  is  objectionable,  and 
the  substitution  of  afternoon  tea  for  a  little  rest  or  sleep 
is  also  unwise. 

5.  Cocoa  is  made  from  the  seeds  of  trees  of  the  genus 
Theobroma,  and  chocolate  is  prepared  from  cocoa.  In  the 
solid  form  both  are  highly  nutritious,  as  shown  by  the 
following  average  results  of  analyses. 

Proteid  Fat         Carbohydrate 

Cocoa 21.6%  28.9%  37.7% 

Chocolate        12.9%  48.7%  30.3% 

When  used  as  a  beverage,  however,  the  nutriment  derived 
from  them  is  small.  In  addition,  cocoa  and  chocolate  both 
contain  theobromine,  a  substance  closely  related  chemically 
to  caffeine  and  possessing  much  the  same  stimulating  prop- 
erties. In  general,  the  same  hygienic  considerations  which 


DBUGS,  ALCOHOL,  AND  TOBACCO  363 

apply  to  the  use  of  tea  and  coffee  should  guide  us  also  in 
the  use  of  chocolate  and  cocoa. 

6.  Soda  Water  and  Similar  Beverages.  —  Of  these  little 
need  be  said.    In  general  they  are  harmless  enough,  espe- 
cially  to    those    enjoying   perfect   digestion.     The   large 
amount  of  sugar  which  they  contain  is  apt  to  make  matters 
worse  in  many  cases  of  dyspepsia;  by  taking  them  fre- 
quently between  meals  the  appetite  for  wholesome  food 
is  impaired,  and  excessive  indulgence  in  them  under  any 
circumstances  is  needless  and  foolish. 

7.  Alcoholic  Beverages.  —  In  the   case  of  an  alcoholic 
drink  we  have  to  deal  with  something  which,  like  tea  and 
coffee  and  cocoa  and  "  temperance  drinks,"  is  used  as  a 
beverage  and  to  that  extent  must  be  classed  in. the  same 
group.    Alcoholic  drinks  are,  however,  taken  as  stimulants 
and  so  resemble  tea  and  coffee  and  cocoa,  but  they  differ 
from  all  of  these  in  their  action  upon  the  body.    Moreover, 
their  abuse  gives  rise  not  only  to  degraded  moral  and 
social  conditions  but  is  also  attended  with  bad  hygienic 
effects.    Every  one  should  be  informed  of  their  nature  and 
of  the  dangers  attending  their  use. 

The  common  alcoholic  beverages  consist  of  (1)  malt 
liquors,  including  beer  and  ale ;  (2)  wines,  such  as  hock, 
claret,  Burgundy,  sherry,  and  champagne;  (3)  distilled 
liquors,  including  brandy,  whisky,  rum,  and  gin ;  and  (4) 
liqueurs  and  cordials.  These  groups  are  distinguished 
from  one  another  largely  by  the  method  of  preparation 
and  by  the  amount  of  alcohol  they  contain.  Malt  liquors 
are  fermented  liquors  which  contain  from  three  to  eight 
per  cent  of  alcohol ;  wines  are  also  fermented  liquors,  but 
contain  from  seven  to  twenty  per  cent  of  alcohol ;  dis- 
tilled liquors,  on  the  other  hand,  are  first  fermented  and 
then  concentrated  by  distillation,  and  contain  from  thirty 
to  sixty-five  per  cent  of  alcohol.  In  all  these  the  most 
important  constituent,  so  far  as  their  physiological  action 


364 


THE  HUMAN  MECHANISM 


FIG.  110.  Yeast  cells 


upon  the  body  is  concerned,  is  the  chemical  compound 
known  as  ethyl  alcohol  (C2H6O  or  C2H5 .  OH). 

8.  Fermentation.  —  The  ethyl  alcohol  in  each  of  these 
beverages  is  produced  by  the  action  of  yeast  on  sugar,  and 
this  action  is  known  as  alcoholic  fermentation.    Yeast  is  a 

unicellular  plant,  and  when  a  small 
amount  of  it  is  added  to  a  solution  of 
grape  sugar  or  fruit  sugar  it  breaks 
up  these  substances,  chiefly  into  al- 
cohol and  carbon  dioxide  gas.  The 
latter  passes  off,  while  the  alcohol 
remains  behind  in  the  solution.  In 
addition  to  these  chief  products  of 
fermentation  there  are  always  formed 
other  products  in  small  quantities, 

and  to  these,  in  part,  the  flavor  of  the  fermented  mixture 
is  due.  Different  varieties  of  yeast  produce  different  kinds 
of  fermentation.  Thus  one  variety  (domesticated  yeast)  is 
used  in  making  beer,  and  another  (wild  yeast)  in  making 
wine.  The  amount  of  alcohol  produced  differs  with  the 
yeast  used,  as  do  also  the  character  and  quantity  of  the 
secondary  products.  The  growth  of  yeast,  like  that  of  all 
living  ferments,  is  checked  by  the  accumulation  of  the 
products  of  its  own  activity.  Consequently  when  the  alco- 
hol produced  reaches  a  certain  percentage  (usually  less 
than  ten  per  cent)  the  fermentation  ceases.  Alcoholic 
drinks  which  contain  higher  percentages  of  alcohol  are  pre- 
pared by  special  processes  which  will  be  described  later. 

9.  Malt   Liquors.  —  Malt  consists    of   sprouted    grains 
(chiefly  barley).    The  grains   contain  a  large   amount  of 
starch  which  during  the  process  of  germination  is  converted 
into  sugar  by  diastase,  an  enzyme  produced  by  the  living 
cells  of  the  plant, — the  action  of  diastase  being  essentially 
similar  to  that  of  the  ptyalin  of  the  saliva.    The  germinat- 
ing plant  thus  comes  to  contain  considerable  quantities  of 


DBUGS,  ALCOHOL,  AND  TOBACCO  365 

sugar,  together  with  salts,  proteids,  and  other  substances. 
The  watery  extract  of  malt  is  known  as  wort,  and  it  is  this 
which,  after  being  boiled  with  hops,  is  acted  upon  by  the 
yeast.  The  liquid  thus  produced  from  wort  by  fermenta- 
tion is  known  as  ale,  beer,  stout,  porter,  etc.,  according  to 
the  conditions  under  which  the  fermentation  takes  place 
and  the  character  of  the  malt  and  the  yeast  employed. 
German  beers  contain  from  three  to  four  per  cent  of  alco- 
hol ;  ale  contains  from  four  to  six  per  cent. 

10.  Wines.  —  Wine  is  produced  by  the  fermentation  of 
the  juice  obtained  by  crushing  grapes,  and  the  yeast  comes 
from  the  "  bloom  "  on  the  skin  of  the  grapes.    The  juice, 
or  "  must,"  thus  extracted  is  allowed  to  undergo  fermenta- 
tion, and  the  fermented  liquid  is  wine.    Most  wines,  how- 
ever, are  subjected  to  subsequent  treatment.    Some  are 
allowed  to  ripen  in  wooden  casks,  during  which  process 
there  take  place   chemical  changes  which  give  to   each 
wine    its    peculiar   flavor.     In    other   cases    the    wine    is 
"  fortified  "  by  the  direct  addition  of  alcohol.    Wines  differ 
from  one  another  according  to  the  variety  of. the  grape 
used  in  making   the   must,  according  to   the  variety  of 
yeast  used  for  fermentation,  and .  according  to  other  cir- 
cumstances. 

11.  Distilled  Liquors  and  Spirits.  —  This  group  of  alco- 
holic beverages  contains  the  highest  percentage  of  alco- 
hol, and  includes  whisky,  brandy,  rum,  and  gin.    In  the 
making  of  all  of  these  the  essential  procedure  is  the  same ; 
namely,   first   to    produce    fermentation   in    some    sugary 
liquid,  and  afterwards  to  distill  from  the  products  of  this 
fermentation  its  alcohol  and  some  other  volatile  constitu- 
ents.   Whisky   is   made  by  distilling   fermented   corn   or 
rye ;  brandy  may  be  spoken  of  as  distilled  wine  ;  rum  is 
distilled  from  fermented  molasses,  and  gin  from  a  fer- 
mented mixture  of  rye  and   malt,  —  juniper  berries  and 
other  substances  being  added  to  the  distilled  product.    In 


366  THE  HUMAN  MECHANISM 

general,  distilled  liquors  contain  from  thirty  to  sixty  per 
cent  of  alcohol. 

With  these  differences  of  preparation,  alcoholic  bever- 
ages differ  greatly  among  themselves,  independently  of  the 
quantity  of  alcohol  they  contain,  and  some  of  their  special 
effects  are  due  to  other  constituents.  The  chief  danger 
of  most  of  them,  however,  lies  in  the  action  of  the  ethyl 
alcohol  upon  the  system,  and  we  shall  confine  our  dis- 
cussion to  the  effects  of  this  substance.  The  problem  is  by 
no  means  a  simple  one,  because  these  beverages  are  used 
in  so  many  different  ways  by  different  people.  Moreover, 
the  results  of  their  use  differ  according  to  the  constitution 
of  the  person  using  them,  and  according  to  his  other  habits 
of  life.  Sweeping  assertions  are  too  frequently  made,  in 
good  faith,  only  to  be  found  false  by  experience  in  special 
cases,  and  in  this  way  harm  is  done  where  good  was  in- 
tended. For  example,  it  is  often  asserted  that  alcohol  used 
in  any  amount  whatever  is  a  poison  to  the  healthy  organ- 
ism. If  this  be  so,  it  is  the  only  known  drug  of  which 
this  is  true.  Dr.  John  J.  Abel,  from  whom  we  shall  ex- 
tensively quote,  says  on  this  subject:  "All  poisons  are 
capable  of  being  taken  without  demonstrable  injury  in  a 
certain  quantity,  which  is  for  each  of  them  a .  special 
though  sometimes  very  minute  fraction  of  their  toxic  or 
lethal  dose.  There  is  no  substance  which  is  always  and 
everywhere  a  poison."  Alcohol  is  a  drug,  and,  like  many 
drugs,  may  be  and  frequently  is  used  in  poisonous  doses ; 
but  it  must  not  be  supposed  that  its  real  danger  lies  in  the 
fact  that  it  always  exerts  a  poisonous  effect  on  the  body. 

12.  The  Physiological  Action  of  Alcohol.  —  As  to  the 
immediate  action  of  alcohol  on  the  body  we  may  say  that 
it  belongs  in  the  same  general  class  of  drugs  as  the  ether 
and  chloroform  used  for  anesthesia ;  in  other  words,  its 
general  action  is  that  of  an  hypnotic  or  anesthetic.  To 


DKUGS,  ALCOHOL,  AND  TOBACCO  367 

quote  again  from  Dr.  Abel :  "  An  exhilarating  action  is 
an  inherent  property  of  these  substances  in  certain  doses. 
Occasionally  the  physician  meets  with  persons  who  have 
formed  the  habit  of  inhaling  chloroform  from  the  palm  of 
the  hand  or  from  a  lightly  saturated  handkerchief.  The 
inhalation  is  usually  carried  on  for  a  short  time  only,  and 
its  object  is  to  induce  a  pleasant  form  of  mental  stimula- 
tion. Only  occasionally  is  the  inhalation  of  chloroform 
carried  on  until  helpless  intoxication  occurs."  And  again : 
"  That  alcohol  can  produce  as  profound  anesthesia  as  any 
of  the  substances  named  is  also  well  known.  In  the  days 
before  anesthesia  it  was  the  custom  of  bone  setters  to  ply 
their  patients  with  alcohol  in  order  to  facilitate  the  reduc- 
tion of  difficult  dislocations.  .  .  .  The  anesthesia  produced 
by  alcohol  is,  however,  not  commendable,  since  i,t  cannot 
safely  be  induced  in  a  short  time  and  is  too  prolonged. 
The  quantity  needed  for  surgical  anesthesia  would  in  many 
cases  lead  to  a  fatal  result." 

13.  Is  Alcohol  a  Stimulant  ?  —  The  view  of  the  action  of 
alcohol  just  stated  is,  of  course,  borne  out  by  the  condition 
of  a  thoroughly  intoxicated  person  ;  but  it  is  opposed  to 
the  very  general  idea  that  alcohol,  except  in  large  doses,  is 
to  be  regarded  as  a  stimulant.  Whether  we  shall  call  it  a 
"stimulant"  or  not  depends  upon  how  we  use  that  term. 
Some  of  the  exhilarating  effects  of  alcoholic  drinks  might 
lead  us  to  speak  of  it  in  this  way.  People  who  have  drunk 
wine  often  become  more  talkative,  so  that  the  first  effects 
of  intoxication  often  resemble  those  of  stimulation.  There 
is,  however,  strong  reason  for  thinking  that  this  action  is 
only  superficially,  and  not  fundamentally,  a  case  of  stimu- 
lation, as  we  shall  now  see. 

In  studying  the  physiology  of  the  nervous  system  we 
found  that  processes  of  inhibition  are  as  important  in  its 
operation  as  are  those. of  excitation;  and  in  mental  opera- 
tions the  course  of  our  thinking  is  constantly  checked  or 


368  THE  HUMAN  MECHANISM 

inhibited  by  the  knowledge  of  facts  opposed  to  the  con- 
clusions towards  which  we  are  tending.  Probably  it  is  this 
essential  feature  of  all  accurate  and  valuable  mental  work 
which  is  the  first  to  be  paralyzed  by  alcohol.  The  man  who 
takes  alcohol  becomes  fluent,  not  because  he  is  stimulated, 
but  because  of  the  removal  of  checks  whose  presence  may 
make  him  talk  less  fluently,  but  which  at  the  same  time 
make  him  speak  more  accurately.  He  may  become  witty, 
and  may  say  some  brilliant  things  ;  but  he  will  almost 
always  do  and  say  some  very  erratic  things. 

The  following  (by  Dr.  Abel)  appears  to  be  a  sound  state- 
ment of  our  present  knowledge  of  this  important  subject  : 
"  Alcohol  is  not  found  by  psychologists  to  increase  the 
quantity  or  vigor  of  mental  operations ;  in  fact,  it  clearly 
tends  to  lessen  the  power  of  clear  and  consecutive  reason- 
ing. In  many  respects  its  action  on  the  higher  functions 
of  the  mind  resembles  that  of  fatigue  of  the  brain,  though 
with  this  action  is  associated  a  tendency  to  greater  motor 
energy  and  ease. 

"In  speaking  of  a  certain  type  of  individual  James  says: 
4  It  is  the  absence  of  scruples,  of  consequences,  of  consider- 
ations, the  extraordinary  simplification  of  each  moment's 
outlook,  that  gives  to  the  explosive  individual  such  motor 
energy  and  ease.'  This  description  aptly  applies  to  the 
individual  who  is  under  the  influence  of  a  4  moderate ' 
quantity  of  alcohol.  It  tends  to  turn  the  inhibitive  type 
of  mind  into  the  4  hair-trigger '  type.  We  have  said  that 
the  speech  and  the  bearing  of  men,  the  play  of  their  fea- 
tures, all  bear  witness  to  the  action  of  alcohol  on  the  brain; 
that  it  removes  restraints,  blunts  too  acute  sensibilities,  dis- 
pels sensations  of  fatigue,  causes  a  certain  type  of  ideas 
and  mental  images  to  follow  each  other  with  greater  rapid- 
ity, and  gives  a  4  cerebral  sense  of  richness.' 

"  Larger  quantities,  such  as  are  for  most  individuals 
represented  by  one  or  two  bottles  of  wine  (ten  per  cent 


DBUGS,  ALCOHOL,  AND  TOBACCO  369 

of  alcohol),  may,  according  to  the  resistance  and  type  of 
individual  in  question,  cause  a  lack  of  control  of  the  emo- 
tions ;  noticeably  affect  the  power  of  attention,  of  clear  judg- 
ment and  reason  ;  and  decidedly  lower  the  acuteness  of  the 
several  senses.  In  many  individuals  such  quantities  will 
develop  so  marked  an  anesthetic  action  that  all  phenomena 
of  intoxication  may  be  seen  to  follow  each  other  in  due 
sequence,  finally  to  end  in  the  sleep  of  drunkenness. 

"  There  has  been  much  discussion  as  to  whether  alcohol 
is  in  any  sense  a  stimulant  for  the  brain.  We  have  seen 
that  pharmacologists  of  high  repute  deny  that  it  has  this 
action,  holding  that  alcohol  is  a  sedative  or  narcotic  sub- 
stance which  belongs  to  the  same  class  as  paraldehyde  and 
chloroform  ;  that  its  stimulating  action  is  but  fictitious ; 
and  that  even  the  earlier  phenomena  of  its  action  are  to 
be  referred  to  a  paralyzing  action  on  cerebral  (inhibitory) 
functions.  This  theory  assumes  an  unequal  action  on 
cerebral  functions  in  the  order  of  time.  Kraepelin,  how- 
ever, holds  that  this  is  a  purely  subjective  analysis,  and 
that  in  the  early  stages  of  its  action  alcohol  truly  stimu- 
lates the  motor  functions  of  the  brain ;  that  a  state  of  men- 
tal exhilaration,  of  '  motor  excitability,'  may  coexist  with 
undiminished  power  of  perception  and  judgment.  His 
psychological  experiments  on  the  action  of  alcohol,  taken 
all  in  all,  do  not,  however,  entirely  prove  his  position." 

Some  cases  of  apparent  stimulation  are  really  due  to  the 
fact  that  alcohol,  when  taken  in  the  form  of  wines  and 
distilled  liquors,  sets  up  an  irritation  in  the  mucous  mem- 
brane of  the  mouth,  oesophagus,  and  stomach,  which  re- 
flexly  excites  the  heart  to  greater  activity  or  for  the  time 
being  reflexly  stimulates  the  nervous  system.  Such  stimu- 
lation is,  however,  transient  and,  as  the  alcohol  is  absorbed 
into  the  blood,  gives  way  to  depression  and  even  stupor. 

It  is  neither  possible  nor  necessary  to  state  here  in  full 
the  reasons  which  have  led  to  what  seems  to  the  authors 


370  THE  HUMAN  MECHANISM 

the  erroneous  view  that  alcohol  in  small  doses  is  a  stimu- 
lant and  only  in  larger  doses  a  depressant  and  hypnotic. 
Enough  has  been  said  to  show  that  there  are  at  least  two 
opinions  about  the  matter:  that  even  if  alcohol  is  at 
times  a  stimulant,  it  is  an  uncertain  stimulant;  and  that 
its  excitation  is  liable  to  give  way  at  any  time  to  de- 
pressing effects.  A  critical  examination  of  the  literature 
on  the  subject  has  failed  to  demonstrate  to  us  a  direct 
stimulating  action  of  alcohol  on  any  of  the  functions,  such 
as  the  beat  of  the  heart,  respiration,  digestion,  etc.  At 
times,  especially  in  sickness,  alcohol  may  be  useful;  but 
the  evidence  tends  to  the  conclusion  that  where  it  exerts 
any  physiological  action  on  the  healthy  body  at  all,  that 
action  is  usually  depressing.  This  is  notably  true  as  to 
the  beat  of  the  heart,  as  to  respiration,  and  as  to  the 
ability  to  do  muscular  work. 

We  have  dwelt  at  length  upon  this  question  in  order  to 
disabuse  the  student's  mind  of  the  idea  that  alcoholic  drinks 
can  be  safely  depended  upon  as  an  aid  in  the  perform- 
ance of  work.  Few  causes  are  more  effective  in  leading 
to  the  abuse  of  alcohol  than  the  idea  that  when  one  finds 
difficulty  in  doing  a  thing  it  may  be  accomplished  more 
easily  by  having  recourse  to  beer  or  wine  or  whisky  for 
their  "  stimulating  "  effect.  In  general,  so  far  is  this  from 
being  the  truth  that  the  person  seeking  such  aid  is  really 
using  a  hypnotic  and  depressant.  Obviously  he  would  be 
acting  more  wisely  to  adopt  other  methods  of  accomplish- 
ing his  end.  Nor  is  this  conclusion  merely  theoretical. 
Brain  workers  who  wish  to  "  keep  a  clear  head  "  almost 
universally  avoid  alcoholic  drinks,  at  least  until  work  is 
over.  And  even  among  those  who  do  drink  it  is  customary 
to  avoid  drinking  until  the  day's  work  is  done. 

14.  Alcohol  in  Muscular  Work. —  That  the  general  effect 
of  alcoholic  drinks  is  to  depress  rather  than  stimulate  the 
powers  of  the  body  is  furthermore  indicated  by  the  results 


DKUGS,  ALCOHOL,  AND  TOBACCO  371 

of  experiments  on  men  doing  heavy  work,  as,  for  example, 
soldiers  on  forced  marches.  In  the  Ashanti  campaign  the 
effect  of  alcohol  as  compared  with  beef  tea  was  tested. 
"  It  was  found  that  when  a  ration  of  rum  was  served  out, 
the  soldier  at  first  marched  more  briskly,  but  after  about 
three  miles  had  been  traversed  the  effect  of  it  seemed  to  be 
worn  off,  and  then  he  lagged  more  than  before.  If  a  second 
ration  were  given,  its  effect  was  less  marked,  and  wore  off 
sooner  than  that  of  the  first.  A  ration  of  beef  tea,  how- 
ever, seemed  to  have  as  great  a  stimulating  power  as  one 
of  rum,  and  not  to  be  followed  by  any  secondary  depres- 
sion "  (Lander  Brunton).  The  results  of  these  and  other 
experiments  lead  us  to  the  conclusion  that  alcohol  cannot 
be  depended  upon  to  increase  the  capacity  for  hard  mus- 
cular work,  and  that  in  the  great  majority  of  cases  it  actu- 
ally diminishes  it. 

15.  The  Dilation  of  Cutaneous  Arteries  by  Alcohol. — 
One  of  the  most  important  effects  of  alcoholic  drinks  is 
the  dilation  of  the  arteries  of  the  skin,  thus  sending  more 
warm  blood  to  the  surface.  It  is  a  common  experience 
among  persons  not  accustomed  to  alcoholic  drinks  that 
even  a  small  amount  "  makes  the  face  hot "  and  flushed, 
and  the  red  face  of  the  toper  is  proverbial.  The  result 
of  this  dilating  effect  is  that  the  temperature  of  the  skin 
rises  and  the  individual  feels  warmer.  Congested  states  of 
internal  organs  may  thus  be  relieved,  and  this  is  probably 
one  reason  why  men  leading  an  exclusively  sedentary  life 
often  use  alcoholic  drinks  apparently  to  some  advantage. 
But  even  these  would  do  infinitely  better  to  secure  the 
same  result  by  proper  muscular  activity. 

Even  if  a  temporary  advantage  appears  to  be  gained 
in  some  cases  or  at  some  times,  this  has  often  to  be  paid 
for  by  bad  secondary  effects,  such  as  impaired  capacity  for 
good  work  some  hours  later ;  and  in  mental  work  of  the 
highest  kind,  such  as  original  writing  or  composition,  the 


372  THE  HUMAN  MECHANISM 

after  effects  of  alcoholic  drinks  are  sometimes  prolonged 
and  easily  detected  by  the  subject  of  the  experiment. 

16.  Alcohol  as  a  Defense  against  Exposure  to  Cold. — Be- 
cause of  this  effect  upon  the  cutaneous  circulation  alco- 
holic drinks  are  frequently  used  by  men  exposed  to  cold, 
with  the  mistaken  idea  that  the  conditions  within  the  body 
are  thereby  improved.    The  student  has,  however,  learned 
(p.  192)  that  a  feeling  or  sensation  of  warmth  does  not 
necessarily  indicate  greater   heat   production  within  the 
body  ;  and  he  also  knows  that  bringing  the  blood  to  the 
skin  when  the  body  is  exposed  to  cold  serves  to  increase 
the  loss  of  heat.    As  a  matter  of  fact,  the  internal  temper- 
ature often  falls  when  alcohol  is  taken  under  these  condi- 
tions.   The   story  is  told  of  some  woodsmen  who   were 
overtaken  by  a  severe  snowstorm  and  had  to  spend  the 
night  away  from  camp  ;  they  had  with  them  a  bottle  of 
whisky,  and,  chilled  to  the  bone,  some  imbibed  freely  while 
others  refused  to  drink.    Those  who  drank  soon  felt  com- 
fortable and  went  to  sleep  in  their  improvised  shelter; 
those  who  did  not  drink  felt  very  uncomfortable  through- 
out the  night  and  could  get  no  sleep,  but  in  the  morning 
they  were  alive  and  able  to  struggle  back  to  camp,  while 
their   companions  who  had    used   alcoholic    drinks  were 
found  frozen  to  death.    They  had  purchased  relief  from 
their  unpleasant  sensations  of  cold  at  the  cost  of  lowering 
their  body  temperature  below  the  safety  point.    This,  if 
true,  was,  of  course,  an  extreme  case  ;  but  it  accords  with 
the  universal  experience  of  arctic  travelers  and  of  lumber- 
men and  hunters  in  northern  woods,  that  the  use  of  alcohol 
during  exposure  to  cold,  although  contributing  greatly  to 
one's  comfort  for  the  time  being,  is  generally  followed  by 
undesirable  or  dangerous  after  effects. 

17.  Alcohol  as  a  Food.  --There  has  been  much  discus- 
sion as  to  whether  alcohol  is  or  is  not  a  food,  i.e.  whether 
its  oxidation  within  the  body  may  supply  energy.    This 


DBUGS,  ALCOHOL,  AND  TOBACCO  373 

question  must  now  be  answered  in  the  affirmative,  although 
whether  it  can  do  more  than  supply  heat  to  maintain  the 
body  temperature,  i.e.  whether  it  can  also  supply  the  power 
for  muscular  work,  as  do  fats  and  carbohydrates,  we  cannot 
in  the  "present  state  of  our  knowledge  positively  say.  In 
many  cases  of  sickness  the  oxidation  of  alcohol  is  probably 
a  useful  source  of  heat  production,  since  it  is  absorbed 
quickly  and  without  digestion;  but  the  healthy  man  does 
not  and  should  not  use  it  in  this  way.  The  amounts  which 
would  be  required  to  be  of  any  considerable  service  as  food 
are  far  beyond  those  in  which  it  may  be  used  with  safety. 
In  other  words,  in  using  alcohol  for  food,  one  would  be 
obtaining  heat  at  the  cost  of  direct  injury  to  many  organs, 
and  also  at  the  cost  of  impaired  working  power.  Moreover, 
men  do  not  use  alcohol  as  a  food ;  they  use  it  as  a  drug. 
So  that  while  the  action  of  alcohol  as  a  food  is  of  practical 
importance  to  the  physician,  who  must  deal  with  the 
abnormal  conditions  of  disease,  its  action  as  a  food  is  not 
a  matter  of  practical  importance  to  healthy  people. 

18.  Pathological  Conditions  Due  to  the  Use  of  Alcohol.  - 
When  alcoholic  beverages  are  taken  in  excessive  amounts 
we  have  the  sad  and  degrading  spectacle  of  a  "  drunken 
spree."  Whether  or  not  the  drinker  at  first  appears  bright 
or  witty,  sooner  or  later  there  is  presented  the  pitiable  pic- 
ture of  complete  loss  of  nervous  coordination  and  control. 
The  man  becomes  silly,  or  maudlin,  or  pugnacious,  as  the 
case  may  be,  but  always  irrational ;  he  staggers,  stumbles, 
or  falls  ;  and  finally  passes  into  a  drunken  stupor.  In  this 
event  the  victim  of  his  own  indulgence  is  said  to  be  "dead" 
drunk,  or  "intoxicated,"  being  as  it  were  thoroughly 
poisoned.  If  such  intoxication  is  frequently  repeated,  there 
is  a  complete  breakdown  of  the  nervous  system ;  the  vic- 
tim of  alcoholic  indulgence  becomes  a  raving  maniac,  and 
with  disordered  vision  thinks  he  sees  all  about  him  snakes 
or  foul  vermin  (delirium  tremens).  The  silly  or  foolish  stage 


374  THE  HUMAN  MECHANISM 

of  this  poisoning  sometimes  provokes  smiles  or  laughter 
in  thoughtless  observers,  but  none  can  witness  the  more 
serious  consequences  of  repeated  intoxication  by  alcoholic 
drinks  without  disgust  and  horror. 

Many  steady  drinkers,  even  though  they  have  never  been 
drunk  in  their  lives,  are  apt  ultimately  to  acquire  various 
diseased  conditions  of  the  body,  into  which  we  cannot 
enter  in  detail.  The  heart  may  be  injured,  or  the  arteries 
become  diseased;  the  repeated  irritation  of  the  stomach 
may  produce  chronic  gastritis  ;  or  the  connective  tissue  of 
the  liver  and  kidneys  may  increase,  thus  crowding  upon 
the  living  cells  and  ultimately  throwing  a  large  part  of 
them  entirely  out  of  use.  While  it  must  not  be  supposed 
that  drinking  alcohol  is  the  sole  cause  of  these  troubles,— 
for  some  or  all  of  them  may  come  from  other  causes, — the 
frequency  of  their  occurrence  in  steady  drinkers  is  sus- 
piciously high,  and  this  has  led  to  the  very  strong  con- 
viction among  medical  men  that  alcohol  plays  a  large  role 
in  producing  them. 

19.  Summary  of  the  Action  of  Alcohol  as  a  Drug.  —  In 
small  doses  alcohol  may  be  completely  oxidized  within  the 
body  without  exerting  any  pharmacological  action.    In  the 
forms  and  amounts  usually  employed  in  alcoholic  beverages 
it  exerts,  in  general,  a  hypnotic  or  anesthetic  action;  the 
result  on  the  system  as  a  whole  depends  on  the  amount 
taken,  and  varies  from  the  paralysis  of  inhibitory  processes 
to   the    depression    of  all    nervous    functions,   ending    in 
drunken  stupor.    Continued  excess  may  produce  exagger- 
ated forms  of  temporary  insanity,  among  which  delirium 
tremens  may  be  mentioned.    There  is,  moreover,  good  rea- 
son for  believing  that  steady  drinking  is  very  frequently 
an  important  agent  in  preparing  the  way  for  many  other 
diseases,  and  is  hence  a  serious  menace  to  health. 

20.  The  Seat  of  the  Danger  in  Alcoholic  Drink.  —  The 
regular  use  of  alcoholic  beverages  is   dangerous  for  the 


DEUGS,  ALCOHOL,  AND  TOBACCO  375 

same  reason  that  the  regular  use  of  any  drug  is  dangerous. 
We  are  too  apt  to  rely  upon  the  drug  to  do  for  us  what  we 
ought  to  accomplish  only  by  the  hygienic  conduct  of  life ; 
the  drug  never  satisfactorily  does  the  work,  and  we  go  from 
bad  to  worse,  and  become  its  slave.  But  there  is  certainly 
greater  danger  in  hypnotic  drugs,  like  alcohol,  than  in  true 
stimulants,  like  coffee,  and  cocoa,  and  tea.  We  need  to 
have  ourselves  well  under  control  when  we  use  any  drug ; 
the  highest  faculties  of  the  mind  must  keep  tight  rein  or 
we  may  lose  control  of  ourselves.  With  hypnotic  drugs  — 
to  which  class  belong  not  only  alcohol  but  ether,  chloro- 
form, opium,  chloral,  etc.  —  there  is  special  danger  that 
these  powers  of  control  (inhibition)  may  be  stealthily  para- 
lyzed before  we  know  it.  Of  course  thousands  of  people 
use  alcohol  in  moderation  and  never  become  drunkards ; 
but  thousands  also,  with  no  intention  of  using  it  to  excess, 
do  unconsciously  let  the  reins  drop,  and  before  they  know 
it  the  drug  gets  the  better  of  them.  Experience  shows 
that  it  is  with  the  hypnotic  drugs  that  this  most  frequently 
happens. 

Again,  if  we  make  a  habit  of  taking  alcoholic  drinks,  we 
are  specially  exposed  to  temptation  from  our  fellow-men  to 
go  too  far.  For  the  most  part,  people  take  coffee  and  tea 
or  do  not  take  them,  as  they  please ;  no  one  urges  them 
to  use  these  drinks  when  they  are  disinclined  to  do  so. 
To  a  less  degree  the  same  thing  is  true  of  tobacco,  although 
here  the  force  of  fashion  and  example  is  stronger.  But 
with  alcoholic  beverages  the  custom  of  "  treating  "  makes 
the  exercise  of  self-restraint  more  difficult  than  it  would 
otherwise  be ;  for  here  we  are  dealing  with  a  drug  which 
is  capable  of  impairing  self-control.  Some  one  "  treats  "  a 
friend  to  a  drink;  the  friend  wishes  to  return  the  compli- 
ment, and  so  they  drink  again;  the  person  with  deficient 
self-control  —  and  what  little  he  has  now  lessened  — 
insists  upon  a  third,  and  so  on,  perhaps  to  intoxication. 


376  THE  HUMAN  MECHANISM 

This,  of  course,  does  not  always  happen;  thousands  are 
strong  and  escape  the  danger,  but  thousands  are  weak  or 
do  not  know  better,  and  many  a  week's  wages  has  gone  in 
this  way,  leaving  behind  poverty  and  misery  and  impaired 
capacity  before  the  close  of  Saturday  night. 

21 .  Concluding  Remarks  on  the  Use  of  Alcoholic  Beverages. 
—  In  the  foregoing  pages  we  have  stated  the  salient  facts 
concerning  the  plrysiological  action  of  alcohol  and  alcoholic 
drinks.    It  only  remains  to  point  out  for  the  student  the 
obvious  conclusions  to  be  drawn  from  them  and  from  the 
long  and   on  the  whole  very  sad  experience  of  the  race 
with  alcoholic  drinks.    The  first  is  that,  except  in  sickness 
and  under  the  advice  of  a  physician,  alcoholic  drinks  are 
wholly  unnecessary,  and  much  more  likely  to  prove  harm- 
ful than  beneficial.    The  last  is  that  their  frequent  and 
especially  their  constant  use  is  attended  with  the  gravest 
danger  to  the  user,  no  matter  how  strong  or  self-controlled 
he  may  be. 

It  is  true  that  history  and  romance  and  poetry  contain 
many  attractive  allusions  to  wine  and  other  alcoholic 
drinks,  and  it  may  also  be  true  that  such  drinks,  by  loosen- 
ing tongues  and  breaking  down  social,  political,  or  other 
barriers  (removing  inhibitions),  may  tend  towards  conviv- 
iality and  good-fellowship ;  but  it  is  no  less  true  that  the 
path  of  history  is  strewn  with  human  wreckage  directly 
due  to  alcohol;  that  many  a  promising  career  has  been 
drowned  in  wine  ;  and  that  indescribable  misery  follows 
in  the  trail  of  drunkenness.  The  only  absolutely  safe  atti- 
tude toward  alcoholic  drinks  is  that  of  total  abstinence 
from  their  use  as  beverages. 

22.  Opium,  Morphine,  and  the  Opium  Habit.  — The  danger 
of  the  use  of  drugs  as  a  regular  habit  of  life  is  perhaps 
most  painfully  illustrated  by  what  is  known  as  the  opium 
habit.    Among  the  most  valuable  remedies  at  the  physician's 
disposal  is  opium  or  its  active  principle,  morphine,  which 


DRUGS,  ALCOHOL,  AND  TOBACCO  377 

possesses  remarkable  power  to  produce  insensibility  to  pain. 
It  sometimes  happens,  however,  that  by  incautiously  using 
this  drug  for  this  purpose  men  and  women  become  addicted 
to  the  habit.  They  finally  cannot  do  without  the  drug, 
and  its  constant  use  causes  an  appalling  moral  and  physical 
degeneration;  so  far  indeed  does  this  often  go  that  the 
victim  will  commit  crime  in  order,  to  obtain  the  drug.  It 
should  be  clearly  understood  that  it  is  unsafe  for  any  one 
to  use  opiates  to  relieve  pain ;  indeed,  these  should  never 
be  used  except  when  prescribed  by  a  careful  physician. 

23.  Chloral,  Cocaine,  etc.  —  Men  and  women  may  become 
slaves  to  the  use  of  other  drugs,  and  in  much  the  same 
way    as    they    become   slaves    to    alcohol   and    morphine. 
Among  these  drugs  are  chloral  and  cocaine.    They  belong 
in  the  same  general  group  of  hypnotics  or  anesthetics,  and 
the  habit  acquired  is  perhaps  no  worse  than  the  opium 
habit.    It  is  certainly  very  little  better.    Let  the  student 
remember  that  the  root  of  the  evil  here,  as  elsewhere,  is 
the  substitution  of  the  use  of  the  drug  for  normal  habits 
of  healthful  living. 

24.  Tobacco.  —  The  physiological  effects  of  tobacco  are 
quite   complicated,  so  complicated  that  it  is  difficult  to 
make  general  statements  with  regard  to  them.    The  effects 
of  chewing  are  quite  different  from  those  of  smoking,  and 
those  of  smoking,  no  doubt,  vary  according  as  the  smoke 
is  or  is  not  drawn  into  the  lungs  (inhaled). 

The  leaf  of  tobacco  contains  a  poison,  nicotine,  which 
exerts  a  powerful  action  on  the  heart  and  on  nerve  cells. 
It  is  not,  however,  proved  that  the  bad  effects  of  the  use 
of  tobacco  are  due  entirely  or  even  chiefly  to  this  substance  ; 
but  it  unquestionably  contributes  to  the  physiological 
effects. 

The  smoke  from  tobacco  also  contains  ammonia  vapor 
which  locally  irritates  the  mucous  membrane  of  the  mouth, 
throat,  nose,  etc.,  and  this  irritating  action  at  times  acts 


378  THE  HUMAN  MECHANISM 

as  a  stimulant  to  the  whole  system  in  much  the  same 
manner  as  do  "  smelling  salts." 

It  has  been  recently  suggested  that,  owing  to  the  incom- 
plete character  of  the  combustion,  tobacco  smoke  contains 
a  small  amount  of  the  poisonous  gas,  carbon  monoxide  (CO), 
and  it  is  quite  possible  that  some  effects  of  smoking  — 
especially  where  the  -smoke  is  drawn  into  the  lungs 
(inhaled)  —  may  be  attributed  to  tin's  gas  ;  but  the  sug- 
gestion has  not  yet  been  submitted  to  the  test  of  actual 
experiment. 

Indeed,  the  physiological  action  of  tobacco  probably  not 
only  varies  with  the  form  in  which  the  tobacco  is  used  but 
is  in  any  case  the  result  of  a  combination  of  a  number  of 
factors  partly  physiological  and  partly  psychical.  We  must, 
here,  however,  confine  our  attention  to  the  purely  hygienic 
aspects  of  the  matter. 

Human  experience  shows  that  the  unwise  use  of  tobacco 
may  unfavorably  affect  digestion,  cause  serious  disorders 
of  the  heart,  and  impair  the  work  of  the  nervous  system. 
Those  training  for  athletic  events  are  usually  forbidden  the 
use  of  tobacco  because  it  "  takes  the  wind,"  i.e.  makes 
impossible  the  most  efficient  training  of  the  heart.  Many 
employers  have  found  that  youths  who  smoke  cigarettes 
are  less  reliable  in  their  work  ;  and  this  is  only  one  instance 
of  the  effect  upon  the  nervous  system  already  referred  to, 
the  same  result  being  observed  in  a  diminished  steadi- 
ness of  the  hand,  often  amounting  to  actual  tremor. 

These  effects  do  not,  of  course,  manifest  themselves  in 
their  extreme  form  whenever  tobacco  is  used ;  but  it  is 
probable  that  they  are  always  present  in  some  degree. 
Whether  they  are  noticeable  or  not  depends  largely  upon 
the  ability  of  the  constitution  to  resist  them.  Tobacco  is 
thus  often  used  without  demonstrable  bad  effects  when  one 
is  leading  a  hygienic  life ;  but  very  often  the  habit,  formed 
under  these  conditions,  persists  after  the  increasing  intensity 


DKUGS,  ALCOHOL,  AND  TOBACCO  379 

of  occupation,  and  the  attendant  cares  and  responsibilities 
of  life  result  in  neglect  of  muscular  exercise  and  improperly 
directed  nervous  activity.  And  as  this  neglect  begins  to 
tell  on  general  health  it  is  found  that  the  unfavorable 
effects  of  tobacco  become  more  pronounced. 

Especially  to  be  condemned  is  its  use  by  those  who 
have  not  attained  their  full  growth.  During  youth  noth- 
ing should  be  allowed  to  interfere  with  the  best  devel- 
opment of  the  heart  and  nervous  system,  and  the  use 
of  tobacco  endangers  the  proper  development  of  both  of 
these  most  important  parts  of  the  human  mechanism.  It 
can  hardly  be  doubted  that  many  a  young  man  has  failed 
to  make  the  most  out  of  life  because  the  habit  contracted 
in  youth  has  struck  in  this  way  at  the  foundations  upon 
which  he  had  subsequently  to  build. 


CHAPTER  XXI 

THE   PREVENTION  AND   CARE   OF  COLDS  AND  SOME 
OTHER  INFLAMMATIONS 

1.  Hygiene  and  Physical  Efficiency.  —  A  most  important 
aim  of  personal  hygiene  is  the  maintenance  of  the  highest 
working  efficiency  of  the  body.    We  should  not  be  content 
with   the  avoidance   of   serious    maladies    like    smallpox, 
diphtheria,  and  consumption,  but  should  try  also  to  avoid 
those  minor  ills  which,  though  temporary  and  rarely  fatal, 
may  seriously  interfere  with  our  capacity  for  usefulness  and 
enjoyment.    The  importance  of  avoiding  constipation  has 
already  been  pointed  out  (p.  128).    The  present  chapter 
will  be  devoted  to  the  practical  consideration  of  such  com- 
mon complaints  as  colds,  rheumatism,  and  diarrhea,  all  of 
which  are  accompanied  by  inflammatory  conditions  in  some 
internal  organ  or  organs,  and  are  favored  by  exposure  to 
cold,  drafts,  or  dampness,  which  chill  the  skin  and  drive 
the  blood  into  the  internal  organs. 

2.  Some  Common  Complaints  and  the  Conditions  which 
favor  them.  —  We  shall  not  give  any  extended  account  of 
the  nature  of  the  complaints  mentioned  in  the  preceding 
paragraph,  for  their  exact  causes  are  still  obscure.    Two 
points,  however,  should  be  emphasized  for  all  of  them. 

1.  TJie  exposure  to  cold  is  not  usually  the  cause  of  these 
diseases  but  only  favors  their  development.  It  is  the  general 
experience  of  arctic  travelers  that  they  suffer  very  little 
or  not  at  all  from  "  colds."  Nansen  and  his  men  were  away 
in  the  Fram  for  more  than  three  years,  and  during  a  large 
part  of  that  time  Nansen  and  Johannson  journeyed  on  sleds 

380 


PREVENTION  AND  CAKE  OF  COLDS          381 

or  afoot,  exposed  to  the  worst  rigors  of  an  arctic  climate. 
At  times,  after  getting  into  their  sleeping  bags,  they  had  to 
thaw  out  their  frozen  clothing  by  the  heat  of  their  own 
bodies  before  they  could  go  to  sleep ;  and  yet  not  one  of 
them  had  "  a  cold "  until  their  return  to  Norway,  when 
an  epidemic  of  colds  broke  out  among  them.  This  and 
numerous  similar  experiences  of  others  suggest  strongly 
that  colds  are  largely  infectious  diseases  ;  but  we  must  not 
forget  that  dampness  and  drafts  are  favoring  conditions 
for  their  development.  The  experience  of  the  race  on  this 
point  is  abundant  and  conclusive. 

2.  Each  of  these  diseases  is  characterized  by  a  condition 
of  inflammation.  We  shall  not  attempt  to  describe  the  exact 
nature  of  inflammation  ;  it  is  sufficient  to  recall  features 
of  it  familiar  to  every  one.  The  sting  of  a  bee  or  hornet 
or  the  bite  of  a  mosquito  results  in  local  inflammation  of  the 
skin  ;  a  severe  case  of  sunburn  presents  a  similar  condition 
over  larger  areas ;  a  wound  of  any  kind  often  shows  more 
or  less  of  the  same  inflammatory  process.  The  part  becomes 
red,  indicating  the  presence  of  an  increased  amount  of 
blood  ;  it  is  swollen,  partly  because  of  the  greater  quantity 
of  blood  and  partly  because  of  the  greater  quantity  of 
lymph  present  in  the  tissue  ;  it  is  usually  hot;  and  it  is 
often  painful.  At  times,  as  in  the  case  of  a  wound  or  boil, 
pus,  or  "  matter,  "  may  be  formed. 

One  or  more  of  these  conditions  is  present  in  an  inflamed 
organ  during  the  diseases  mentioned .  When  we  have  a 
cold  in  the  head  (rhinitis)  the  vascular  membrane  lining 
the  nasal  cavity  is  the  seat  of  trouble  ;  in  a  sore  throat  it 
is  the  pharynx  and  larynx  (pharyngitis  and  laryngitis) ; 
in  a  cold  on  the  chest  (bronchitis)  it  is  the  ciliated  mem- 
brane of  the  trachea  and  bronchi ;  similarly  in  catarrhal 
attacks  of  the  stomach  and  intestine  it  is  the  mucous 
membrane  of  these  organs  ;  and  we  must  think  of  these 
inflamed  tissues  of  the  respiratory  and  alimentary  tracts  as 


382  THE  HUMAN  MECHANISM 

presenting  somewhat  the  same  condition  as  that  seen  in  the 
skin  during  a  bad  case  of  sunburn.  They  all  have  an  exces- 
sive amount  of  blood  within  them  ;  they  are  more  or  less 
swollen,  —  as  when  one's  "nose  is  stopped  up  ";  there  is 
an  unusual  amount  of  fluid  in  the  tissue ;  and  there  is, 
besides,  generally  a  transudation  of  this  fluid  to  the  surface, 
as  in  the  "  running  of  the  nose." 

3.  Congestion  during  Inflammation.  — The  presence  of  an 
excessive  quantity  of  blood  in  the  capillaries  of  an  organ 
is  known  as  "  congestion"  ;  and  this  may  be  of  two  kinds, 

—  active  (or  arterial),  due  to  an  excessive  supply  from  the 
arterial  reservoir ;  or  passive  (or  venous),  due  to  some  inter- 
ference with  the  outflow  into  the  veins.1 

In  a  cold,  congestion  of  the  inflamed  area  begins  as  an 
active  congestion  ;  the  arteries  are  widened,  the  pressure  in 
the  capillaries  is  increased,  and  the  blood  flows  much  more 
rapidly.  This  is  essentially  the  same  thing  —  only  in  greater 
degree  —  that  occurs  when  the  arterioles  of  the  stomach 
dilate  during  digestion,  or  those  of  the  skin  during  exposure 
to  warmth.  This  initial  vascular  stage  is  succeeded  by  one 
of  passive  congestion,  caused  by  the  adhesion  of  white  blood 
corpuscles  to  the  capillary  walls,  thereby  diminishing  the 
bore  of  the  tube,  and  so  making  difficult  the  outflow  into  the 
veins ;  the  velocity  of  the  blood  through  the  capillaries  is 
lessened,  pressure  within  them  is  increased  (why?),  and  they 
become  gorged  with  blood.  Such  is  the  vascular  condition 
in  an  organ  when  an  inflammatory  process  is  at  its  height ; 
the  characteristic  feature  is  the  narrowing  of  the  outlet  of  the 
capillaries  and  the  consequent  excess  of  pressure  within  them. 

4.  Dangers  connected  with  Congestion. —  A  decidedly  con- 
gested condition  is  undesirable  because  it  is  a  predisposing 

1  The  artificial  model  described  on  page  142  may  easily  be  used  to  show 
the  difference  between  arterial  and  venous  congestion.  With  the  nozzle 
in  the  far  end  of  the  rubber  tube,  the  tube  may  be  congested  (or  swollen) 
with  water  by  more  rapid  pumping  (active  congestion)  or  by  narrowing 
the  outlet  (passive  congestion). 


PREVENTION  AND  CAKE  OF  COLDS          383 

cause  of  these  inflammatory  diseases.  It  is  not  the  only  cause 
nor  the  exciting  cause  of  the  disease  ;  but  a  congested  organ 
may  succumb  to  an  attack  of  disease  and  so  become  readily 
inflamed  where  it  would  have  escaped  had  its  vascular  con- 
dition been  normal.  For  example,  the  normal  intestine  may 
be  the  seat  of  some  unusual  bacterial  action  (see  Chapter 
VIII,  p.  125)  and  suffer  no  damage  therefrom,  while  the 
same  bacterial  action  may  give  rise  to  catarrhal  inflamma- 
tion, accompanied  by  diarrhea,  if  it  occurs  when  the  in- 
testinal blood  vessels  are  congested.  Or  again,  whatever 
the  cause  of  an  ordinary  cold  may  be,  bacterial  or  other- 
wise, it  is  probable  that  its  attack  upon  the  perfectly  nor- 
mal organism  may  be  and  frequently  is  resisted ;  while  at 
another  time  a  congested  condition  of  the  nose,  the  throat, 
or  the  bronchial  tubes  may  permit  the  disease  to  gain  a 
foothold  at  that  point.  In  other  words,  the  congestion 
alone  will  not  cause  colds  in  the  head  or  on  the  chest, 
or  diarrheal  troubles  in  the  intestine  ;  something  else  is 
needed.  We  may  have  the  congestion  without  the  cold, 
and  we  may  also  succumb  to  a  cold  without  the  preliminary 
congestion ;  but  the  presence  of  congestion  often  presents 
to  an  infecting  agent  the  weak  spot  which  is  needed  in  order 
that  the  latter  shall  secure  a  foothold  and  do  damage. 

5.  The  Avoidance  of  Congestion  during  Colds,  etc.  The 
Care  of  Catarrhal  Conditions.  —  Again,  whenever  an  inflam- 
matory process  is  established,  there  is,  as  we  have  seen,  more 
or  less  of  passive  congestion ;  under  these  circumstances 
everything  should  be  done  to  avoid  arterial  dilation  in  the 
inflamed  area.  Suppose  there  is  catarrhal  inflammation  of 
some  part  of  the  small  intestine,  accompanied  by  diarrhea; 
the  outlet  into  the  veins  is  narrowed  and  there  is  conse- 
quently more  or  less  "  backing  up  "  of  the  blood  in  the 
capillaries  (passive  congestion).  This  congestion  is  kept 
within  moderate  limits  so  long  as  the  arterioles  maintain 
a  good  tonic  constriction  and  so  limit  the  amount  of  blood 


384 


THE  HUMAN  MECHANISM 


which  can  flow  in;  if,  however,  they  are  made  to  dilate 
widely,  —  by  eating  a  hearty  meal  for  example,  —  this 
check  is  removed,  blood  flows  in  under  high  pressure,  and 
the  congestion  is  increased.  Hence  in  all  such  catarrhal 


FIG.  111.  Experiment  to  show  the  effect  of  arterial  constriction  in  relieving 
capillary  congestion 

In  the  upper  figure,  constriction  of  the  tube  at  A  results  in  distention  (con- 
gestion) of  the  tube  between  the  pump  and  the  fingers;  if,  however,  the 
tube  be  also  constricted  at  B,  as  in  the  lower  figure,  pressure  falls  between 
A  and  B,  and  the  congestion  is  relieved. 

attacks  the  diet  should  be  very  light  and  preferably  con- 
fined to  those  things  which  are  easily  digested  and  absorbed. 
6.  The  Care  of  Colds,  etc.  — Again,  when  suffering  from 
any  of  these  inflammatory  diseases  of  internal  organs  the 
greatest  care  should  be  taken  to  avoid  chilling  the  skin, 
because  this  means  (Chapter  XII)  compensating  dilation  in 
the  inflamed  area,  and  therefore  increase  of  congestion  there. 
One  should  be  warmly  clad  (not  overclad) ;  the  living  and 
sleeping  rooms,  though  well  ventilated,  should  not  be  cold ; 
when  a  cold  sleeping  room  cannot  be  avoided  some  cover- 
ing for  the  head  is  often  useful,  as  this  part  of  the  body  is 
not  protected  by  the  bed  covering ;  cold  baths  should  be 


PEEYEKTION  AND  CAEE  OF  COLDS         385 

discontinued;  and,  above  all,  dampness  should  be  avoided. 
In  severe  cases  it  is  often  necessary  for  the  patient  to  go 
away  from  a  damp  climate  to  a  dry  one.  The  key  to  the  situ- 
ation, so  far  as  the  management  of  the  circulation  is  con- 
cerned, consists  in  keeping  in  the  skin  its  full  share  of  blood. 
A  brief  chilling  of  even  a  comparatively  small  area  of  the 
skin  (e.g.  cold  feet)  may  produce  a  congestion  in  the  inflamed 
organ  capable  of  undoing  the  healing  work  of  hours  or  days. 

A  word  must  be  said  in  this  connection  about  the  "  fresh- 
air"  cure  for  colds,  etc.  There  is  no  doubt  that  being  in 
the  fresh,  dry  air,  even  if  it  is  cold  air,  and  preferably  out 
of  doors,  is  better  for  a  cold  or  any  other  catarrhal  condition 
than  remaining  in  a  closed  room.  But  this  should  never 
involve  the  chilling  of  any  portion  of  the  skin ;  one  should 
be  warmly  clad,  even  the  head  and  neck  being  well  pro- 
tected. It  makes  little  or  no  difference  that  we  breathe 
cold  air ;  but  it  makes  a  very  great  difference  whether  or 
not  the  skin  is  exposed  to  cold  air. 

In  taking  care  of  colds  and  similar  troubles  it  is  well 
to  remember  that  the  inflammation  is  only  one  of  the  un- 
favorable conditions  against  which  the  system  is  struggling. 
Consequently  we  should  not  expect  the  disease  to  yield  in 
all  cases  to  our  measures  for  keeping  the  skin  warm.  At 
times  a  hot  bath,  a  drink  of  hot  lemonade,  or  other  meas- 
ures for  bringing  the  blood  to  the  skin  checks  a  threatened 
cold;  but  none  of  these  measures  is  of  great  value  after 
the  disease  has  once  obtained  a  foothold.  It  is  then  a 
struggle  between  the  body  and  the  disease  ;  and  we  can 
do  more  by  merely  avoiding  the  chilling  of  the  skin  than 
by  taking  measures  to  produce  marked  cutaneous  dilation. 
The  true  policy,  in  other  words,  is  to  give  the  living  body 
every  chance  to  cure  itself,  and  this  is  best  done  by  not 
calling  on  it  to  do  too  many  other  things  at  the  same 
time.  Thus  muscular  exercise,  ordinarily  one  of  our  best 
means  of  keeping  the  blood  in  the  skin,  is  not  usually 


386  THE  HUMAN  MECHANISM 

advisable  when  a  cold  is  at  its  height,  because  an  added 
strain  would  thereby  be  imposed  on  the  already  sorely 
taxed  system.  Later,  when  the  worst  is  over,  it  is  a  valuable 
aid,  though  it  should  not  be  too  vigorous  until  one  is  on 
the  road  to  complete  recovery. 

"  Stuff  a  cold  and  starve  a  fever  "  is  one  of  those  pithy 
sayings  whose  very  pith  may  be  poisonous.  A  full  meal 
when  we  have  a  cold  in  the  head  often  clears  up  the  nasal 
congestion  for  a  time  (probably  by  drawing  the  blood  to 
the  stomach  and  intestine)  and  so  deludes  us  into  suppos- 
ing that  our  ".  stuffing  "  has  done  good.  It  may  also,  and 
doubtless  often  does,  support  and  reenforce  the  body  in  its 
battle  with  the  disease.  What  it  may  do,  however,  is  to 
overtax  the  body  with  the  digestion  of  a  heavy  meal ;  the 
meal  may  not  be  properly  digested;  -bacterial  processes  in 
the  excessive  mass  of  food  may  produce  abnormal  and 
poisonous  substances  (see  Chapter  VIII)  which  gain  admis- 
sion to  the  blood,  and  the  "  last  state  "  of  the  patient  may 
be  "  worse  than  the  first." 

7.  Measures  for  stopping  Colds.  —  When  one  "  feels  a 
cold  coming  on,"  that  is,  early  in  the  struggle,  active 
measures  should  first  be  taken  to  dilate  the  blood  vessels 
of  the  skin.  A  hot  bath  before  going  to  bed,  and  hot  drinks 
such  as  hot  lemonade,  may  be  tried.  If  the  cold  does  not 
promptly  yield  to  these  measures,  rest  in  bed  is  usually  the 
best  treatment.  The  nervous  system  is  frequently  in  no 
condition  to  sustain  hard  work  of  any  kind,  and  hence, 
until  the  cold  begins  to  clear  up,  it  is  well  to  confine  the 
diet  to  easily  digestible  foods  in  moderate  quantity,  and 
to  remain  very  quiet.  Few  people,  unfortunately,  act  on 
this  principle.  "  It 's  only  a  cold  "  is  made  the  excuse  for 
meeting  every  engagement  that  may  have  been  made,  or 
for  attempting  to  do  full  work.  Sometimes,  perhaps  gener- 
ally, no  serious  results  follow,  but  at  other  times  the  penalty 
is  heavy. 


PREVENTION  AND  CARE  OF  COLDS         387 

Very  often  a  cold  is  a  more  serious  matter  than  we 
suppose.  Only  physicians  appreciate  how  often  it  is  the 
sign  of  more  serious  disease.1  While  we  cannot  say  that 
one  should  always  stop  work  until  the  cold  is  overcome, 
we  do  say  that  limiting  work  to  the  minimum  and  secur- 
ing all  the  rest  possible  is  always  advisable  and  should  be 
the  rule  rather  than  the  exception.  We  may,  unknown 
to  ourselves,  be  nursing  more  than  a  cold ;  and,  even  if 
we  are  not,  we  always  hasten  the  cure  by  taking  care  of 
ourselves. 

8.  The  Use  of  Drugs  for  Catarrhal  Conditions. —  A  remedy 
very  frequently  resorted  to  for  colds  and  other  inflamma- 
tory troubles  is  the  taking  of  some  drug.  Large  fortunes 
have  been  made  by  the  sale  of  "  cough  medicines "  and 
the  like.  Some  of  these  "  remedies  "  are  worse  than  use- 
less ;  others  may  do  no  harm,  and  some  may  be  useful. 
But  none  of  them  are  cures.  The  cure  of  the  cold  is  effected 
not  by  the  drug  but  by  the  system  of  the  patient ;  the  drug 
can  do  no  more  than  remove  some  condition  which  stands 
in  the  way  of  the  healing  effort  of  the  organism.  The 
severe  coughing  of  a  bad  case  of  bronchitis  is  often  irritat- 
ing to  the  inflamed  surface  of  the  air  passages,  and  may 
stand  in  the  way  of  clearing  up  the  inflammation.  Here 
a  drug  may  do  good,  though  it  should  be  taken  only  on 
the  advice  of  a  physician,  and  never  on  the  strength  of 
newspaper  testimonials  to  its  wonderful  virtues.  But  the 
use  of  these  medicines  does  not  render  unnecessary  the 
measures  we  have  outlined  as  the  proper  treatment.  It 
is  worse  than  foolish  to  dose  oneself  with  drugs  when  a 
cold  is  coming  on,  and  then  attempt  to  do  full  work ;  often 
the  only  result  of  such  folly  is  a  complete  "  knocking  out 
of  the  stomach  "  by  the  drug.  The  average  "  cough  medi- 
cine "  is  especially  likely  to  do  this. 

1  In  typhoid  fever  a  "cold  on  the  chest"  is  frequently  the  first  outward 
indication  of  the  disease. 


388  THE  HUMAN  MECHANISM 

9.  The  Belief  in  Drugs. —  A  century  ago  the  attitude 
of  men  and  women  toward  practical  hygiene  consisted 
largely  in  living  in  ignorance  of  the  workings  of  the  body, 
taking  little  or  no  care  of  it,  and  then  whenever  bad  feel- 
ings appeared  "taking  something  simple"  to  cure  them. 
This  course  of  conduct  was  persisted  in  until  something 
happened,  —  and  something  usually  did  happen,  sooner 
rather  than  later, — when  recourse  was  had  at  once  to  drugs. 
The  doctor  was  the  man  who  knew  what  drug  to  "  give  " 
for  each  disease.  He  was  expected  to  "prescribe " ;  and  if  he 
did  not  prescribe  something,  he  failed  to  satisfy  his  patient, 
who  concluded  that  the  physician  did  not  understand  the 
disease.  The  attitude  of  the  public  was  largely  that  of 
neglecting  personal,  individual  care  of  the  health  and  mean- 
time implicitly  believing  that,  no  matter  what  happened, 
some  drug  could  be  swallowed  which  would  set  matters 
right. 

Medicine,  and  especially  personal  hygiene,  have  now 
advanced  beyond  this  crude  condition.  To-day  we  realize 
as  never  before  that  the  individual  is  responsible  for  the 
intelligent  care  of  his  health.  The  time  is  probably  coming 
when  he  will  be  held  as  responsible  for  the  care  of  his  body 
as  he  is  to-day  for  the  care  of  his  morals.  At  the  same 
time,  drugs  are  being  much  less  used  by  the  best  physicians. 
It  is  not  true  that  all  drugs  are  useless ;  quite  the  con- 
trary ;  but  it  is  true  that  careful  nursing  often  counts  for 
more  than  does  the  use  of  drugs.  Typhoid  fever  is  to-day 
often  treated  with  no  drugs  at  all,  and  the  tendency  to  use 
drugs  in  other  diseases  is  distinctly  lessening. 

The  wise  physician  is  often  hampered  in  his  work  by  the 
survival  of  this  old-fashioned  belief  in  the  all-sufficiency 
of  drugs.  Instances  of  it  are  sometimes  encountered  even 
among  otherwise  intelligent  people.  We  should  under- 
stand that  in  all  cases  of  illness  the  one  treatment  which 
should  be  applied  is  good  nursing,  whether  by  a  trained 


PKEVENTION  AND  CAKE  OF  COLDS         389 

nurse,  or  by  one's  family,  or  by  oneself.  If  medicine  is 
given,  it  is  usually  subsidiary  to  the  main  procedure,  al- 
though sometimes,  as  in  the  antitoxin  treatment  of  diph- 
theria, it  is  the  main  thing.  But  in  no  case  should  we  be 
so  foolish,  so  unreasonable,  as  to  distrust  or  lose  confidence 
in  a  physician  because  he  gives  few  drugs  or  none  at  all. 

10.  The  Avoidance  of  Colds,  etc.    General  Principles.  —  If 
the  care  of  these  slight  ailments  is  of  importance,  their 
prevention  is  of  much  greater  importance.    And  first  of 
all  among  preventive  measures  must  be  placed  not  the 
avoidance  of  drafts  and  other  chilling  of  the  skin,  not 
clothing,    but    the  proper    hygienic    care    of  the    body,  — 
regular  and  sufficient  muscular   exercise,  the   avoidance 
of  improper  feeding  (for  colds  are  often  due  to  digestive 
disturbances  resulting  from  overfeeding),  and  good  habits 
of  sleep  and  rest.    When  these  things  are  properly  attended 
to,  one  may  usually  suffer  considerable  chilling  of  the  skin 
without  ill  effect.    It  is  not  true,  as  is  often  asserted,  that 
by  attention  to  these  general  matters  the   protection  of 
the   body  from  exposure   to    cold   becomes    unnecessary, 
but  it  is  true  that  without  this  .attention  such  protective 
measures  are  apt  to  be  of  little  avail. 

Among  general  measures  none  is  more  important  than 
the  avoidance  of  exposure  to  chilling  influences  when  the 
nervous  system  is  depressed  by  marked  fatigue.  We  take 
cold  more  readily,  much  as  we  are  more  susceptible  to  any 
disease,  when  we  are  tired.  It  is  a  question  of  a  struggle 
between  the  organism  and  unfavorable  external  or  internal 
conditions,  and,  in  general,  the  greater  the  fatigue  of  the 
organism  the  less  is  its  chance  of  success  in  the  struggle. 

11.  The  Avoidance  of  Colds.    Special  Measures.  —  As  to 
measures  specially  concerned  with  the  avoidance  of  con- 
gestion in  internal  organs,  let  us  first  state  clearly  the 
principle  involved  and  then  pass  to  its  practical  applica- 
tions.   The  condition  to  avoid  is  the  undue  constriction  of 


390  THE  HUMAN  MECHANISM 

the  blood  vessels  of  the  skin,  produced  by  chilling.  The 
danger  is  not  in  the  mere  exposure  to  cold;  people  may 
be  comparatively  lightly  clad  on  a  cold  dry  day,  when 
there  is  no  wind,  without  chilling  the  skin,  because  the 
layer  of  air  in  contact  with  the  skin  becomes  warmed,  and 
in  the  absence  of  wind  even  light  clothing,  if  dry,  suffices 
to  keep  this  warm  air  in  contact  with  the  body.  But  if 
the  air  is  damp,  so  that  it  readily  conducts  heat,  or  if  the 
wind  is  blowing,  so  that  convection  becomes  important,  or 
if  the  body  is  near  cold  objects  to  which  it  can  radiate  its 
heat,  the  skin  may  be  easily  chilled,  especially  if  we  are 
making  no  muscular  exertion  (pp.  193-198). 

Again,  during  muscular  exertion  exposure  to  cold  is 
usually  harmless,  even  if  the  clothing  be  light,  because 
the  increased  heat  production  within  the  body  results  in 
an  adequate  flow  of  warm  blood  through  the  skin.  We 
seldom  take  cold  during  vigorous  muscular  work  on  a 
cold  day.  It  is  when  we  are  sitting  still  or,  even  more, 
when  we  are  lying  down  and  the  muscles  are  relaxed,  that 
we  should  be  on  our  guard. 

Finally,  and  most  important  of  all,  is  the  fact  that  the 
dangerous  region  of  atmospheric  temperature  is  confined 
to  the  narrow  limits  of  a  few  degrees  just  below  the  proper 
room  temperature.  This  proper  room  temperature  for  light 
clothing  is  about  66°  F.  with  low  or  normal  atmospheric 
humidity,  and  about  69°  or  70°  F.  for  high  humidity. 
Above  these  points  there  is  no  chilling  of  the  skin.  Five 
or  ten  degrees  below  these  points  we  feel  so  cold  that  we 
become  uncomfortable  and  take  steps  to  remedy  matters, 
either  by  putting  on  heavier  clothing  or  by  heating  the 
room.  It  is  when  the  temperature  is  only  slightly  below 
what  it  should  be  that  we  are  apt  to  be  unaware  of  the 
insidious  increase  of  arterial  constriction  and  chilling  in 
the  skin,  until,  after  an  hour  or  more  of  it,  we  suddenly 
awake  to  the  true  condition  of  affairs.  This  is  apt  to 


PREVENTION  AND  CAKE  OF  COLDS          391 

happen  when  the  fire  in  the  stove  or  in  an  open  grate 
goes  down.  It  also  happens  at  times  when  we  come  from 
a  walk  out  of  doors  into  a  room  of  this  "  dangerous 
temperature,"  say  63°  F.  on  a  day  of  high  humidity;  the 
skin  is  warmed  by  the  exercise  we  have  been  taking,  so 
that,  as  we  enter  the  room,  it  does  not  seem  cold  (for  the 
temperature  we  really  notice  is  that  of  the  skin,  not  that 
of  the  room  at  all) ;  on  sitting  still  in  the  room  the  cuta- 
neous dilation  of  muscular  exercise  passes  off  so  gradually 
that  we  do  not  notice  the  change,  and,  before  we  are  aware 
that  we  are  chilly,  marked  internal  congestion  may  have 
been  produced. 

It  is  also  well  to  remember  that  not  all  parts  of  the 
room  have  the  same  temperature.  The  floor  is  colder  than 
the  ceiling ;  it  is  colder  nearer  exposed  walls  and  windows 
than  away  from  them,  and  the  common  habit  of  sitting  near 
a  window  on  a  cold  day  while  reading  or  sewing  is  not  wise. 

12.  Cooling  off  suddenly.  —  It  is  an  old  saying  that  it  is 
not  well  to  u  cool  off  suddenly."  While  there  is  some 
truth  in  this,  it  is  not  true  in  general,  nor  in  the  form 
stated.  It  is  perfectly  safe  for  most  healthy  people  to 
take  a  cold  bath  after  exercise,  or  to  pass  directly  from  a 
hot  bath  into  a  cold  one  (see  Chapter  XXIV).  The  sudden 
cooling  which  experience  has  found  to  be  harmful  is  where 
the  clothing  has  been  saturated  with  perspiration  and  one 
cools  off  by  sitting  still  in  a  breeze  or  in  a  cool  place.  Here 
the  clothing  remains  damp  and  so  conducts  heat  readily 
from  the  skin  ;  and  the  danger  lies  not  in  the  cooling  off 
but  in  the  prolonged  chilling  process  which  follows  it. 
Consequently  it  is  a  general  rule  that  clothing  made  damp 
by  rain  or  perspiration  should  be  changed  as  soon  as  pos- 
sible, or  else  that  drafts  and  cold  rooms  should  be  avoided 
until  the  clothing  is  dry. 

It  is  unnecessary  to  multiply  examples.  In  all  the  prin- 
ciple is  the  same,  —  the  avoidance  of  conditions  which 


392  THE  HUMAN  MECHANISM 

produce  marked  constriction  of  cutaneous  blood  vessels, 
with  the  accompanying  congestion  of  internal  organs  ;  and 
the  student  is  again  reminded  that  by  this  course  we  do 
not  always  secure  immunity  from  internal  inflammations ; 
we  merely  remove  one  of  the  conditions  which  favor  their 
development. 

13.  "  Hardening  "  the  System  to  Cold.  —  We  must  refer 
briefly  to  the  importance  of  what  is  popularly  known  as 
hardening  the  system  to  cold.  Cold  unquestionably  pro- 
duces its  effects  in  some  people  more  readily  than  in 
others,  and  these  differences  are  largely  dependent  upon 
habit  or  training.  When  the  living  rooms  are  kept  above 
70°,  and  heavy  clothing  is  always  worn  out  of  doors,  the 
skin  is  constantly  subjected  to  a  tropical  climate  and 
becomes  more  sensitive  to  external  cold.  Internal  conges- 
tion will  then  be  produced  at  67°  or  68°  F.  which  would 
not  take  place  above  60°  or  62°  F.  in  persons  who  have 
been  accustomed  to  cold.  In  other  words,  it  is  possible  to 
overdo  the  matter  of  protection  from  external  cold.  For 
this  reason,  overheated  rooms  and  the  use  of  heavy  wraps 
while  walking  in  moderately  cold  weather  (30°- 50°  F.) 
are  very  objectionable. 

We  should  thus  harden  ourselves  to  cold ;  but  it  should 
never  be  forgotten  that  this  process  of  hardening  may  be 
carried  too  far.  To  harden  oneself  does  not  mean  that  the 
temperature  of  the  living  room  should  be  kept  below  65°  F., 
nor  that  sleeping  rooms  should  be  cold  enough  to  freeze 
water  at  night.  Severe  colds  and  rheumatism  have  been 
contracted  by  this  folly. 

Many  people  fail  to  realize  that  because  a  little  will 
do  good,  it  does  not  necessarily  follow  that  more  will  do 
better.  One  person  is  impressed  by  the  undoubted  fact 
that  it  is  possible  to  eat  too  much  meat,  and  thereupon 
abstains  from  meat  altogether;  another  discovers  that  a 
sedentary  life  is  a  bad  thing,  and  hastens  intemperately 


PREVENTION  AND  CAKE  OF  COLDS         393 

to  take  "  century  rides  "  on  a  wheel.  One  finds  that  he  has 
been  overclad,  and,  discarding  all  warm  clothing,  shivers 
throughout  the  winter;  another,  on  learning  the  possi- 
ble value  of  cold  bathing,  enthusiastically  but  unwisely 
plunges  into  the  coldest  water  he  can  get,  and  stays  in  it 
until  his  skin  is  blue.  Very  likely  any  one  of  these  exam- 
ples can  be  duplicated  from  the  reader's  own  circle  of  ac- 
quaintances. It  is  important  to  remember  that  "  nothing 
too  much  "  is  always  a  good  rule,  and  nowhere  is  it  more 
essential  than  in  the  hygienic  conduct  of  life. 

14.  Reasons  for  avoiding  Colds  and  Other  Inflammatory 
Troubles.  —  We  may  conclude  this  chapter  with  some  facts 
showing  the  hygienic  importance  of  the  prevention  of  colds 
and  other  inflammatory  diseases,  such  as  sciatica,  lumbago, 
and  rheumatism  in  its  various  forms.  In  all  these  diseases 
we  find  the  same  close  connection  between  the  chilling 
of  the  skin  and  the  onset  of  the  disease,  so  that  what  has 
been  especially  urged  with  regard  to  colds  applies  in  large 
measure  to  the  entire  group.  But  some  may  say,  "  These 
are  slight  ailments;  why  not  ignore  and  disregard  them?" 

1.  The  first  and  sufficient  reason  is  that  these  ailments 
interfere  seriously  with  our  working  power  and  with  our 
capacity  for  usefulness  and  enjoyment.    Every  one  knows 
from  experience  that  the  body  is  not  as  good  a  machine 
during  the  progress  of  a  cold  or  a  diarrheal  attack,  or 
while   suffering  from  sciatica   or  slight  attacks  of  rheu- 
matism.   We  should  strive  not  only  to  live,  but  to  live 
well;  not  merely  to  do  things,  but  to  do  them  with  our 
might ;  not  merely  to  live  and  work,  but  to  live  happily 
and  to  work  cheerfully. 

2.  The   popular  impression  as   to   the  frequency  with 
which  pulmonary  consumption,  pneumonia,  etc.,  are  pre- 
ceded by  common  colds  is  much  exaggerated.     It  is  never- 
theless probable  that  in  some  cases  a  cold  is  the  means 
of  lowering  the  power  of  resistance  to  the  more  serious 


394  THE  HUMAN  MECHANISM 

disease,  and  we  should  take  every  reasonable  precaution 
which  will  maintain  the  ability  of  the  body  to  cope  success- 
fully with  the  inroads  of  diseases,  especially  of  those  for 
which  there  is  no  certain  cure. 

3.  Colds  and  similar  troubles  have  a  well-known  tendency 
to  become  chronic.  Probably  no  sufferer  from  nasal  catarrh, 
or  chronic  bronchitis,  or  chronic  diarrhea,  if  he  had  his 
life  to  live  over  again,  would  neglect  measures  tending  to 
avoid  the  occurrence  of  these  conditions.  Only  those  who 
do  not  know  from  experience  the  capacity  of  such  troubles 
to  produce  annoyance  and  discomfort  can  regard  their  pre- 
vention as  unworthy  of  serious  attention.  We  cannot  too 
strongly  emphasize  the  fact  that  chronic  troubles  are  very 
frequently  the  result  of  the  repetitions  of  the  neglected 
inflammations  which  accompany  the  acute  attack ;  they  are 
due  not  so  much  to  inherent  weakness  of  the  tissue  or 
organ  as  to  the  carelessness  of  the  individual  about  avoid- 
ing them,  or  the  failure  to  give  them  the  attention  they 
deserve  when  they  occur.  One  of  our  leading  physicians, 
a  man  of  the  widest  experience  and  soundest  judgment, 
writes,  concerning  chronic  nasal  catarrh,  "  It  is  sad  to 
think  of  the  misery  which  has  been  entailed  upon  thou- 
sands of  people  owing  to  the  neglect  of  nasopharyngeal 
catarrh  by  parents  and  physicians." 


CHAPTER  XXII 
THE  CARE  OF  THE  EYES  AND  EARS 

The  visual  apparatus  (eye,  optic  nerve,  nerve  endings, 
etc.)  furnishes  one  of  the  most  important  paths  from  the 
world  without  to  the  brain  within,  and  it  is  of  the  utmost 
importance  to  the  exercise  of  the  highest  functions  of  the 
human  mechanism  that  this  path  be  kept  as  smooth  as 
possible.  Unfortunately,  however,  the  path  is  seldom  either 
straight  or  smooth,  and  it  frequently  presents  serious  ob- 
stacles. The  curvature  of  the  cornea  or  of  the  lens  may 
be  irregular  ;  the  muscle  of  accommodation  may  be  weak  ; 
the  retina  may  be  too  near  or  too  far  from  the  lens,  or 
its  sensitive  cells  may  too  readily  become  fatigued  by  the 
stimulation  of  light ;  finally,  the  path  into  the  brain  may 
be  made  of  poorly  constructed  nervous  tissue,  or  in  the 
brain  itself  the  coordinations  upon  which  depend  our 
visual  judgments  (p.  256)  may  be  imperfect.  The  simplest 
act  of  vision  is  the  end  result  of  a  most  complicated  series 
of  events,  difficulty  with  any  one  of  which  may  make  quick 
and  accurate  seeing  impossible.  Many  a  child  has  been 
considered  stupid  simply  because  an  unrecognized  condi- 
tion of  myopia  or  astigmatism  renders  it  impossible  to  read 
clearly  the  printed  page  or  the  distant  blackboard;  and 
many  people,  adults  as  well  as  children,  suffer  from  head- 
aches and  other  troubles  because  of  the  strain  thrown  on  the 
nervous  system  in  the  effort  to  work  with  defective  vision. 

When  one  is  leading  an  outdoor  life,  occupied  in  the 
work  of  the  farm  or  the  lumber  camp,  and  doing  but  little 
reading,  the  eyes  usually  give  little  trouble,  because  it  is 

395 


396  THE  HITMAN  MECHANISM 

only  when  looking  at  near  objects  (three  feet  or  less  away) 
that  the  mechanism  of  accommodation  is  called  into  vigor- 
ous action.  Eye  strain  is  usually  produced  by  prolonged 
near  work  with  eyes  incapable  of  enduring  without  un- 
due fatigue  what  is  demanded  of  them.  Hence  it  is  that 
defects  of  vision  are  more  common  to-day  than  they  were 
a  hundred  years  ago.  Both  the  vocations  and  the  avoca- 
tions of  modern  life,  with  their  large  amount  of  reading, 
writing,  and  other  forms  of  near  work,  impose  upon  the 
eye  the  most  trying  and  difficult  task  it  can  be  called  upon 
to  perform.  The  use  of  glasses  is  more  common  than  for- 
merly, and  the  care  of  the  eyes  is  forced  upon  us  as  an 
important  factor  in  the  hygienic  conduct  of  life. 

1.  The  Necessity  of  Expert  Advice.  —  In  the  care  of 
the  eyes  expert  advice  is  indispensable.  The  detection  of 
defects  of  vision  frequently  demands  the  best  skill  of  those 
who  are  thoroughly  acquainted  with  the  physiology  of  the 
entire  visual  apparatus,  including  its  relation  to  other  bodily 
functions,  and  who  are  also  provided  with  every  means  for 
gaining  an  insight  into  the  conditions  which  are  giving 
trouble.  The  selection  of  the  proper  glass,  for  example, 
when  lenses  are  needed  is  more  than  a  mere  matter  of  test- 
ing vision  with  test  cards  ;  and  eyes  may  be  seriously 
injured  by  using  glasses  prescribed  on  the  basis  of  informa- 
tion gained  by  imperfect  methods. 

First  of  all,  then,  let  us  insist  upon  the  necessity  of 
competent  medical  advice  whenever  there  is  reason  to  sus- 
pect something  wrong  with  the  eyes.  If  vision  is  not  dis- 
tinct, if  the  eyes  tire  quickly  when  used  for  near  work, 
and  even  when  one  suffers  from  headaches,  "  nervousness" 
and  other  forms  of  malaise  without  apparent  cause,  it  is 
wise  to  find  out  whether  some  remediable  defect  of  vision 
is  not  at  the  root  of  the  trouble. 

On  first  thought  it  may  seem  unreasonable  to  consult  an 
oculist  with  regard  to  headaches  or  other  troubles  with 


CARE  OF  EYES  AND  EARS        397 

organs  having  no  obvious  connection  with  the  eye;  but  when 
we  remember  the  fact  that  all  parts  of  the  central  nervous 
system  are  connected  with  one  another,  it  is  easy  to  see 
how  undue  strain  of  one  part  in  the  effort  to  see  with 
astigmatic  or  otherwise  defective  eyes  may,  by  injuriously 
affecting  other  parts  of  the  brain  or  spinal  cord,  unfavor- 
ably influence  organs  which  themselves  have  nothing  to  do 
with  vision.  Over  and  over  again  it  happens  that  head- 
aches and  other  troubles  are  relieved,  as  by  magic,  when 
vision  is  made  perfect  by  the  use  of  proper  glasses. 

With  these  remarks  as  to  the  importance  of  skilled 
advice  in  the  -care  of  the  eyes,  we  may  pass  to  those  prac- 
tical measures  which  should  be  under  the  intelligent  indi- 
vidual control  of  every  man  and  woman.  Suppose  vision 
is  perfect,  or  as  nearly  perfect  as  the  best  of  medical  skill 
can  make  it;  what  precautions  in  the  use  of  the  eyes 
favor  the  maintenance  of  their  best  working  condition  ? 

2.  Resting  the  Eyes.  —  First  of  all  we  would  suggest 
the  importance  of  resting  the  eyes  now  and  then  while 
engaged  in  near  work.    This  is  accomplished  by  the  sim- 
ple expedient  of  looking  for  a  few  moments  at  some  dis- 
tant object  (p.  250).    The  brief  relaxation  of  the  effort  of 
accommodation    does    for  the   neuromuscular  mechanism 
involved  exactly  what  a  brief  relaxation  of  the  body  in 
sleep  accomplishes  for  the  body  as  a  whole. 

3.  Illumination  of  the  Object.    The  Importance  of  Con- 
trast. —  The  ease  with  which  the  details  of  an  object  are 
seen  depends  chiefly  on  the  contrasts  of  shade  and  color 
which  these  details  present    to  the  eye,  and  nothing  so 
influences    this  contrast  as  the   amount  of  illumination. 
Thus  as  the  light  fades  in  the  evening,  the  white  paper  of 
a  printed  page  becomes  darker  and  darker,  until  finally  it 
reflects  to  the  eye  little  more  light  than  the  black  ink  of 
the  printed  letters,  which  consequently  no  longer  stand 
out  clear  and    distinct.    In  order  to  admit  all  the  light 


398  THE  HUMAN  MECHANISM 

possible,  the  pupil  enlarges,  and  in  so  doing  lessens  the 
distinctness  of  the  retinal  image  (spherical  aberration); 
more  important  than  this,  we  hold  the  page  closer  to  the 
eye,  thereby  enlarging  the  retinal  image  and  increasing  the 
intensity  of  stimulation,  but  throwing  far  more  work  upon 
the  ciliary  muscle  to  focus  for  the  near  object.  All  of  these 
unfavorable  conditions  taken  together  place  undue  strain 
upon  the  mechanism  of  accommodation. 

Hardly  less  objectionable  is  excessive  illumination  of 
an  object.  After  a  certain  intensity  of  light  is  reached,  the 
retina  no  longer  responds  to  increase  of  stimulation  with 
increase  of  visual,  reaction.  If  there  were  in  addition  to 
our  sun  a  second  sun  which  sent  into  the  eye  twice  as 
much  light,  the  second  sun  would  seem  no  brighter  than 
the  first  because  the  effect  of  the  first  upon  the  eye  has 
already  passed  the  point  which  calls  forth  the  greatest 
possible  reaction  in  the  retina.  To  apply  this  principle  to 
the  case  in  point,  we  have  only  to  remember  that  a  printed 
letter  is  not  absolutely  "  dead  black,"  but  reflects  some 
light.  When  the  illumination  is  moderate  this  reflected 
light  hardly  affects  the  retina  at  all,  and  the  contrast 
between  the  black  letter  and  the  white  paper  is  marked. 
As  the  intensity  of  illumination  increases,  however,  the 
effect  upon  the  retina  of  the  light  coming  from  the  letters 
increases  more  rapidly  than  the  effect  of  that  coming  from 
the  paper.  Contrast  is  lessened  and  sharper  accommoda- 
tion as  well  as  closer  attention  is  needed  to  see  distinctly. 
Added  to  this,  no  doubt,  is  the  fatigue  and  lack  of  sensi- 
tiveness in  the  retina,  resulting  from  overstimulation. 

4.  The  Size  of  Type.  —  The  use  of  fine  type  should  be 
reduced  to  a  minimum,  because  it  necessitates  greater 
effort  of  accommodation  and  intensifies  all  the  evils  of 
improper  illumination.  Any  printed  matter  which  must 
be  held  less  than  eighteen  inches  from  the  eye  in  order  to 
be  seen  clearly  is  undesirable  for  long-continued  reading. 


CARE  OF  EYES  AND  EARS        399 

Especially  is  this  true  in  youth,  since  then  the  eye  is  more 
plastic,  and  excessive  strain  of  the  muscle  of  accommoda- 
tion, pulling  as  it  does  on  the  sclerotic  and  the  choroid 
coats,  may  lead  to  permanent  deformation  of  the  curved 
surfaces.  The  marked  increase  of  myopia  within  the  past 
forty  or  fifty  years  is  explained  in  this  way. 

5.  Highly  Calendered  Paper  Objectionable.  —  Closely  con- 
nected with  the  size  of  the  type  is  the  character  of  the  paper 
on  which  it  is  printed.    This  should  be  as  dull  as  possible 
in  order  to  avoid  the  confusing  effect  of  a  glossy  surface. 
The  use  of  highly  calendered  paper  in  many  books  and 
serial  publications,  because  such  paper  lends  itself  more 
readily  to  the  reproduction  of  pictures  in  half  tone,  is  a 
sacrifice  of  hygienic  considerations  to  cheapness. 

6.  Importance  of  a  Steady  Light.    Reading  on  Railroad 
Trains.  —  The  source  of  illumination  for  near  work  should 
be  as  free  as  possible  from  unsteadiness  or  nicker,  since  a 
flickering  light  necessitates  the  most  accurate  accommo- 
dation.   A  "  student's  lamp,"  "  Rochester  burner,"  or  incan- 
descent electric  lamp  is  preferable  in  this  respect  to  candles, 
gas  jets,  and  arc  lights  for  near  work. 

For  the  same  reason  caution  is  demanded  in  the  matter 
of  reading  on  railroad  trains.  American  railway  trains 
have  recently  become  so  heavy,  and  the  roadbed,  rails, 
etc.,  have  been  so  much  improved  in  various  ways,  that 
the  danger  of  reading  or  writing  while  traveling  by  rail  is 
much  less  than  formerly.  At  the  same  time  the  danger 
still  exists,  and  reading  on  many  railway  and  trolley  cars 
is  still  to  be  done  with  caution,  or,  better  still,  avoided 
altogether. 

7.  Microscopes,  Telescopes,  and  other  optical  instruments 
require  close  and  sometimes  continuous  use  of  one  or  both 
eyes,  and    are   popularly   supposed   to  be  "hard    on  the 
eyes."    But  this  is  not  necessarily  the  case,  except  for 
beginners  and  investigators;  for  beginners,  because  they 


400  THE  HUMAN  MECHANISM 

try  to  see  clearly  by  focusing  with  the  eye  rather  than 
with  the  use  of  the  focusing  apparatus  of  the  instrument ; 
for  investigators,  because  the  eyes  are  used  for  too  long 
periods  at  a  time.  Optical  instruments  are  easily  focused, 
and,  if  care  be  taken  to  provide  good  lighting,  routine  work 
with  them  need  not  be  specially  trying  to  the  eyes. 

8.  The  Removal  of  Cinders.  —  Particles  of  dust,  cinders, 
etc.,  are  often  washed  away  from  the  surface  of  the  eyeball 
by  the  copious  secretion  of  tears  which  they  call  forth. 
Sometimes,  however,  they  must  be  removed  directly  from 
the  eyeball  or  the  inner  surface  of  the  eyelid.    In  the  case 
of  the  lower  lid  this  operation  presents  little  difficulty,  for 
the  eyelashes  of  this  lid  are  easily  seized,  the  lid  drawn 
forward  away  from  the  eyeball,  and  the  surfaces   of  the 
eyelid  and  eyeball  readily  inspected.    If  any  foreign  body 
is  there  located,  it  may  be  removed  by  the  corner  of  a 
handkerchief.    Successful  manipulation  of  the  upper  lid 
is  more  difficult,  because  a  piece  of  cartilage  immediately 
above  the  eyelashes  interferes  with  turning  back  the  lid. 
The  gaze  of  the  patient  should  be  directed  downward,  a 
small  pencil  or  other  cylindrical  object  pressed  against  the 
upper  portion  of  the  lid,  above  the  cartilage,  the  eyelashes 
seized,  and  the  lid  turned  upwards  and  backwards  over  the 
pencil. 

9.  Recapitulation.    The  Care  of  the  Eyes.  —  To  summa- 
rize, we  may  remind  the  student  that  the  eyes,  no  less 
than  other  organs,  should  be  kept  sound  and  strong  by 
attention  to  the  general  health  and  welfare  of  the  body. 
Work,  play,  rest  and  sleep,  muscular  exercise,  wise  feeding, 
and  regular  removal  of  the  wastes,  —  these  and  all  other 
general  hygienic  habits  help  to  keep  the  eyes  sound  and 
strong ;  but  besides  these,  posture  in  work,  lighting,  paper 
(not  forgetting  wall  paper),  printing,  dust,  cinders,  smoke, 
acid  fumes,  traveling,  sight-seeing,  and  many  other  con- 
ditions have  their  effect.    Finally,  it  must  not  be  forgotten 


CAEE  OF  EYES  AND  EAES        401 

that  the  eyes  are  too  precious  to  be  trifled  with,  and  that 
if  one  has  sore  or  weak  eyes,  or  pain  in  the  eyes,  or  cannot 
see  clearly  to  read  or  to  write,  or  cannot 'plainly  distinguish 
things  near  or  at  a  distance,  then  it  is  always  best  to  con- 
sult an  oculist  or  the  family  physician  for  advice.  Remedies 
or  doctors  puffed  in  high-sounding  advertisements  should 
be  carefully  avoided. 

10.  The  Care  of  the  Ears.  —  Besides  good  care  of  the 
general  health,  which  common  sense  dictates  and  which  we 
have  repeatedly  urged  as  the  fundamental  requirement  in 
the  hygiene  of  all  organs,  there  is  but  little  which  the 
individual  can  do  for  the  ears.    Deafness,  especially  total 
deafness,  is  a  defect  or  injury  perhaps  no  less  serious  than 
blindness.    Acute  hearing  is  probably  as  valuable  as  acute 
vision,  and  a  partial  loss  of  hearing  is  a  handicap  often 
harder  to  overcome  than  are  some  defects  of  vision. 

Keeping  in  mind  the  auditory  apparatus  and  its  connec- 
tions (Chapter  XIV),  it  is  easy  to  see  that  the  drum  may 
be  pierced  or  otherwise  injured  by  slender  objects  thrust 
in  from  without ;  that  catarrh  of  the  throat  may  easily 
extend  into  the  Eustachian  tube,  inflaming  it  or  choking 
its  lumen  or  outlet ;  and  that  any  thickening  of  the  drum 
must  tend  to  make  its  vibrations  slower  and  more  diffi- 
cult. In  these  possibilities  we  have  some  of  the  actual 
causes  of  deafness,  and  none  of  them  is  of  a  kind  to  be 
treated  by  the  patient.  Any  recognition  of  incipient  deaf- 
ness in  oneself  should  be  regarded  as  cause  for  consulting 
a  good  physician.  No  attention  should  be  paid  to  adver- 
tisements promising  to  relieve  deafness,  for  these  are  usu- 
ally traps  calculated  to  catch  the  ignorant,  unwary,  or 
credulous.  It  is  dangerous  to  explore  the  outer  ears  with 
hairpins  or  other  pointed  objects,  as  the  drum  may  thus 
be  broken  or  other  harm  done. 

11.  Noise,  though  delighted  in  by  savages,  who  beat 
tom-toms,  blow  conch  shells,  or  otherwise  tickle  the  sense 


402  THE  HUMAN  MECHANISM 

of  hearing,  and  though  in  moderation  often  found  stimu- 
lating and  enjoyable  by  persons  who  have  been  living  in 
solitude  or  isolation,  is  by  adults  among  the  most  highly 
civilized  peoples  more  and  more  regarded  as  a  necessary 
evil,  or  even  as  a  nuisance.  Children,  on  the  other  hand, 
often  delight  in  noise,  and  horn  blowing,  firecrackers,  and 
pistol  firing  on  holidays  like  the  Fourth  of  July  appear 
to  give  to  them  as  much  pleasure  as  to  their  elders  pain. 
Adults  also  on  occasions  of  rejoicing  still  ring  bells,  beat 
drums,  blow  horns,  and  fire  cannon  in  order  to  express 
their  emotions.  Loud  noise,  like  strong  light,  is  unques- 
tionably stimulating  and  exciting,  and  for  these  reasons, 
though  justifiable  at  times  of  rejoicing,  is  something  to 
be  ordinarily  avoided  as  far  as  possible  in  city  life,  itself 
already  much  too  stimulating  and  exciting.  One  can,  in- 
deed, often  learn  to  sleep  even  in  the  presence  of  dis- 
tracting noises  such  as  those  of  a  busy  city  street;  but 
such  sleep  cannot  possibly  be  as  wholesome  as  that  enjoyed 
in  quiet  places.  The  constant  whistling  of  locomotives, 
which  was  formerly  a  great  nuisance  in  many  American 
cities  and  towns,  has  been  largely  done  away  with,  and  the 
tendency  of  the  times  is  to  cultivate  quiet,  not  only  as  a 
private  luxury  but  also  as  a  public  necessity. 


CHAPTER   XXIII 
THE  HYGIENE  OF  THE  FEET 

The  hygienic  care  of  the  feet  consists  essentially  in 
maintaining  the  ability  of  those  organs  to  bear  easily  and 
without  discomfort  the  weight  of  the  body.  "  Weak  feet " 
are  to  blame  for  many  unhealthful  conditions ;  the  dis- 
comfort or  pain  which  they  cause  as  one  goes  about  the 
ordinary  occupations  of  life  subjects  their  possessor  to 


FIG.  112.  Bones  of  the  right  foot 
Seen  from  the  outer  side 

nervous  strain  and  often  prevents  the  enjoyment  of  that 
muscular  activity  which  the  maintenance  of  health  requires. 
Nor  is  it  generally  known  that  this  state  of  affairs  may  be 
very  largely  avoided  by  intelligent  care.  In  the  majority 
of  cases  weakness  of  foot  is  the  result  of  maltreatment  of 
the  foot,  and  not  the  result  of  inborn  structural  defects. 

Each  foot  consists  of  no  less  than  twenty-six  small  bones 
joined  by  ligaments  and  held  in  proper  position  relative  to 
one  another  by  the  action  of  a  number  of  muscles.  The 
key  to  the  understanding  of  the  hygiene  of  the  foot  is  the 
fact  that  it  is  upon  the  proper  performance  of  the  work 
of  these  muscles  that  the  strength  of  the  foot  primarily 

403 


404  THE  HUMAN  MECHANISM 

depends,  and  that  the  weakening  of  the  foot  is  due  to 
interference  with  their  action,  chiefly  by  the  use  of  wrongly 
shaped  shoes. 

1.  The  Arches  of  the  Foot.  —  The  bones  of  the  foot  should 
form  two  well-marked  arches.  One  of  these  is  the  con- 
spicuous arch  of  the  instep  and  the  other  a  less  conspicuous 
but  important  transverse  arch  immediately  back  of  the  toes. 
Not  only  is  the  preservation  of  these  arches  important 
because  they  help  to  relieve  the  joints  above  them  of  jar, 
but  also  because  under  them  lie  nerves,  blood  vessels,  lym- 
phatics, and  other  tissues  which  are  injured  when  the  arch 


FIG.  113.  Longitudinal  section  through  the  bones  of  the  foot 

Showing  the  arch  of  the  instep  and  the  attachment  of  the  tendon  of  the  calf 
muscle  to  the  heel  hone 

gives  way  and  permits  pressure  upon  them  from  above. 
Figure  144  shows  the  action  of  one  of  the  groups  of  mus- 
cles which  maintain  the  arch  of  the  instep,  and  illustrates, 
in  principle,  how  muscular  action  keeps  the  bones  in  proper 
relative  positions.  The  muscles  shown  in  the  figure  (the 
short  flexors  of  the  toes)  act  like  the  string  of  a  bow,  and 
by  contracting  resist  the  tendency  of  the  weight  of  the 
body  to  break  the  arch  down.  Other  groups  of  muscles 
are  concerned,  but  it  is  unnecessary  that  we  go  into  the 
details  of  their  action.  Enough  has  been  said  to  show 
the  importance  of  keeping  these  muscles  strong,  so  that 
they  may  do  the  work  imposed  upon  them. 


HYGIENE  OF  THE  FEET  405 

The  groups  of  muscles  specially  concerned  are  those 
which  move  the  toes,  and  these,  like  other  muscles,  can  be 
kept  strong  only  by  use.  Consequently  interference  with 
the  freedom  of  action  of  the  toes  must  lead  to  the  dis- 
use and  partial  degeneration  (weakening)  of  the  muscles 
in  question.  The  fundamental  principle  in  the  care  of  the 
foot  is  none  other  than  the  maintenance  of  the  freedom  of 
motion  of  the  toes,  together  with  the  use  of  the  toes  as  well  as 
the  ankle  in  locomotion. 

2.  The  Foot  of  the  Infant  and  the  Adult  Foot.  —  Every 
human  being  begins  life  with  a  foot  possessing  wide  range 


FIG.  114.  Ligaments  of  the  foot  and  ankle 

of  movement,  amounting  almost  to  a  grasping  power.  It 
is  most  instructive  to  watch  a  baby  use  its  toes ;  not  only 
are  they  bent  downward  or  upward  (plantar  and  dorsal 
flexion)  and  spread  apart  (abducted)  with  the  greatest  ease, 
but  in  walking  the  toes  fairly  grasp,  or  dig  into,  the  ground. 
The  adult  foot  of  civilized  man  usually  presents  a  painful 
contrast  to  this.  Generally  the  toes  are  crammed  together, 
their  power  of  spreading  apart  is  wholly  lost,  and  their 
movements  take  no  part  whatever  in  walking.  The  foot, 
in  other  words,  is  reduced  almost  to  the  condition  of  a 
shoemaker's  last. 

Nor  is  this  a  natural  change  due  to  growth  and  develop- 
ment.   It  is  produced  by  the  use  of  shoes  which  permit  no 


406  THE  HUMAN  MECHANISM 

movement  of  the  toes  and  therefore  lead  to  disuse  of  the 
muscles  in  question.  Walking  thus  comes  to  be  performed 
almost  entirely  with  muscles  which  act  upon  the  ankle 
joint,  the  one  articulation  of  the  foot  at  which  movement 
is  still  possible  ;  whereas  had  the  toes  been  allowed  perfect 
freedom  of  action,  the  work  of  lifting  the  weight  of  the 
body  from  the  ground  with  each  step  would  have  been 
shared  by  both  groups  of  muscles,  —  those  which  raise  the 
heel  and  those  which  flex  the  toes.  That  this  is  true  is 
shown  by  the  feet  of  people  who  have  not  worn  constrict- 
ing shoes,  for  in  their  case  the  toes  are  moved  freely  and 
perform  an  important  share  in  locomotion.1 

If  it  be  asked  why  the  flexors  of  the  toes  as  well  as  the 
extensors  of  the  ankle  should  take  part  in  the  act  of  walk- 
ing, the  answer  is  that  it  is  precisely  the  disuse  of  the 
former  which  leads  to  their  degeneration,  so  that  they  are 
no  longer  efficient  in  opposing  the  tendency  of  the  weight 
of  the  body  to  break  down  the  arches  of  the  foot.  It  is 
true  that  "  flat  foot "  is  not  the  invariable  result  of  this 
disuse,  because  some  people  are  so  fortunate  as  to  possess 
ligaments  of  sufficient  strength  to  hold  the  bones  together 
despite  the  pressure  of  this  weight,  and  also  because  tightly 
fitting  shoes  often  assist  in  holding  the  bones  in  position. 
But  it  is  also  true  that  many  others  are  not  so  fortunate ; 
one  or  both  arches  give  way,  and  some  suffer  agony  as  the 
result.  Even  if  the  arches  do  not  break  down,  the  foot  is . 
generally  unable  to  stand  the  strain  of  prolonged  walking 


1  The  action  of  each  of  these  groups  of  muscles  may  be  made  clear  as 
follows  :  with  the  bare  feet  take  a  step  forward  by  first  raising  the  heel 
and  then  pushing  off  by  bending  the  toes  downward  as  far  as  possible. 
It  will  be  found  that  this  second  movement  is  capable  of  assisting  to  a 
very  considerable  extent  in  pushing  the  body  forward.  The  student 
should  thus  make  himself  practically  familiar  with  the  difference  between 
(1)  walking  when  only  the  heel  is  raised  and  the  toes  passively  bent  up- 
ward as  the  step  is  completed,  and  (2)  walking  when  the  raising  of  the 
heel  is  followed  by  the  active  contraction  of  the  plantar  flexors  of  the  toes. 


HYGIENE  OF  THE  FEET 


407 


without  marked  discomfort,  and  it  is  not  too  much  to  say 
that  this  weakness  of  the  foot  is  one  of  the  chief  reasons 
why  most  people  regard  a  walk  of  ten  or  twelve  miles  as 
a  great  task. 

The  hygienic  care  of  the  foot  in  actual  practice  consists 
(1)  in  the  use  of  properly  fitting  shoes,  (2)  in  avoiding  all 


FIG.  115.  Correct  and  incorrect  shapes  of  the  sole  of  the  shoe 

Outline  of  the  sole  in  solid  lines ;  of  the  natural  shape  of  the  foot  in  dotted 
lines.  A,  correct  shape;  in'.B  the  shape  is  correct  except  that  the  median 
line  of  the  sole  is  not  straight  in  the  region  of  the  toes,  thus  pressing  the 
great  toe  over  toward  the  other  toes ;  (7  has  not  only  this  defect  hut  is  tocr 
n  arrow 

interference  with  the  circulation  of  blood  in  the  foot,  (3)  in 
maintaining  proper  conditions  of  temperature  and  moisture 
within  the  shoe,  and  (4)  in  the  training  and  use  of  the 
muscles  of  the  foot,  so  as  to  keep  them  functionally  strong 
and  active. 

3.  Shoes.  —  Among  the  most  important  requirements  of 
a  good  shoe  are  the  following:    (1)  The  sole  of  the  shoe 


408  THE  HUMAN  MECHANISM 

should  everywhere  be  as  wide  as  the  sole  of  the  foot  when 
one  is  standing  and  the  feet  are  warm.  (2)  The  heels 
should  be  low  and  broad.  (3)  The  sole  and  uppers  should 
be  sufficiently  flexible  to  permit  without  great  resistance 
the  bending  of  the  foot  at  the  transverse  line  of  articula- 
tion of  the  toes  with  the  instep.  Many  shoes,  otherwise 
correct,  are  faulty  in  that  the  sole  or  the  upper  from  the 
heel  forward  is  too  stiff  to  permit  the  efficient  action  of  the 
toe  movements.  (4)  The  inner  (or  median)  side  of  the  shoe 
should  be  "  straight,"  i.e.  the  prolongation  of  the  median 
line  of  the  great  toe  should  touch  the  heel.  Figure  115 
shows  the  proper  and  improper  shape  of  the  shoe  in  this 
respect.  Unless  the  foot  is  already  greatly  deformed,  no 
shoe  should  be  tolerated  which  does  not  permit  the  great 
toe,  and  for  that  matter  all  the  toes,  to  point  straight 
forward,  since  otherwise  it  is  not  easy  to  flex  them.  Not 
only  is  the  "  toothpick  "  shoe  a  hygienic  abomination,  but 
any  shoe  in  which  the  inner  or  median  side  slopes  outward 
toward  the  toe  is  highly  objectionable  (see  Fig.  115,  B). 
(5)  In  the  region  of  the  toes  the  shoes  should  have  sufficient 
room  to  permit  perfect  freedom  of  motion  in  the  toes. 

4.  Shoes  for  Deformed  Feet. — It  must  be  frankly  admitted 
that  shoes  which  fill  all  the  above  requirements  are  uncom- 
fortable to  many  feet.  But  this  is  only  because  such  feet 
have  already  become  deformed.  In  such  cases  the  attempt 
should  be  made  to  bring  the  foot  back  toward  its  normal 
shape  by  gradually  approaching  the  lines  above  indicated. 
With  some  hopelessly  deformed  feet  this  is,  of  course, 
impossible;  but  with  many  others  great  improvement  is 
possible. 

Upon  one  point  there  can  be  no  yielding :  children  should 
wear  only  properly  shaped  shoes.  It  is  a  pitiable  sight  to 
see  the  foot  of  a  child,  broadening  out  as  it  does  toward 
the  mobile  toes,  forced  into  a  shoe  which  seems  to  be  con- 
structed on  the  assumption  that  nature  ought  to  have  made 


HYGIENE  OF  THE  FEET  409 

the  human  foot  wedge-shaped  and  that  it  is  man's  part  to 
improve  on  nature. 

Recent  years  have  witnessed  marked  improvement  in 
the  shape  of  shoes.  Fortunately  it  is  now  possible  in  many 
places  to  buy  properly  made  shoes ;  but  further  improve- 
ment is  still  possible,  both  among  those  who  make  shoes 
and  those  who  buy  them.  As  a  matter  of  common  sense 
nothing  could  be  more  absurd  than  the  custom  of  changing 
the  shape  of  shoes  each  season  merely  to  bring  out  a  new 
style;  nor  would  this  be  done  if  people  were  more  gener- 
ally informed  as  to  the  requirements  of  a  good  shoe  and 
insisted  on  having  only  those  which  meet  these  require- 
ments. In  so  far  only  as  there  is  a  general  demand  for 
such  shoes  in  any  community,  will  manufacturers  supply 
them.  The  remedy  lies  with  the  public  rather  than  with 
the  manufacturers. 

And  this  same  public  must  learn  that  neither  from  the 
hygienic  nor  from  the  aesthetic  standpoint  is  a  small  foot 
with  a  pointed  toe  and  high  heel  the  ideal  foot.  Such  is 
not  the  foot  of  the  Apollo  Belvedere  nor  that  of  the  Venus 
of  Melos.  It  is  simply  a  deformity,  belonging  in  the  same 
category  with  the  constricted  waist,  and  far  more  harm- 
ful to  its  possessor  than  the  ear  or  nose  ornaments  of  the 
Hottentot.  No  hygienic  lesson  is  more  important  than 
that  clothing  should  fit  the  body,  and  not  the  body  the 
clothing. 

5.  Interference  with  the  Circulation  in  the  Foot.  —  By 
lacing  the  shoe  too  tightly,  especially  around  the  top, 
and  by  the  use  of  tight  garters  the  superficial  veins  which 
bring  blood  back  from  portions  of  the  foot  are  often  com- 
pressed. More  or  less  of  passive  congestion  results,  and 
this  not  only  produces  discomfort  but  introduces  in  other 
ways  conditions  highly  unfavorable  for  the  free  action  of 
the  foot;  consequently  it  is  part  of  the  hygiene  of  that 
organ  to  avoid  these  congestions  at  all  times.  Garters 


410  THE  HUMAN  MECHANISM 

should  always  be  adjustable  in  length  to  the  size  of  the 
leg,  and  shoes  should  not  be  laced  tightly. 

6.  Proper  Conditions  of  Temperature  and  Moisture  with- 
in the  Shoe.  —  Although  the  best  of  shoes  are  but  poorly 
adapted  to  care  for  the  perspiration  and  to  maintain  an 
equable  temperature  of  the  foot,  some  shoes  are  preferable 
to  others  in  these  respects.  Thus  any  "patent"  or  "enamel" 
leather  is  objectionable  for  walking  because  it  is  almost 
impervious  to  moisture.    In  such  shoes  the  foot  becomes 
overheated  while  one  is  walking,  because  the  perspiration 
does  not  evaporate  from  its  surface ;  and  if  one  afterwards 
sits  still,  the  feet  are  apt  to  become  cold  because  the  wet 
stockings  make  a  good  conductor  of  heat.    Because  their 
surface  radiates  heat  with  such  ease  these  shoes  are  cold 
in  cold  weather;  and  because  they  prevent  the  evapora- 
tion of  perspiration  they  are  hot  in  hot  weather.    While 
unobjectionable    generally   for   dress    occasions,  they  are 
wholly  unsuitable  for  ordinary  wear. 

The  "russet"  shoe  for  summer  wear  is  a  great  boon. 
The  leather  of  which  it  is  made  is  as  porous  as  any  on  the 
market,  and,  because  of  its  color,  absorbs  less  heat  in  warm 
weather.  The  failure  of  the  attempt  a  few  years  ago  to 
retire  these  shoes  from  sale  is  good  evidence  that  people 
can  get  a  certain  shoe  if  only  they  insist  upon  having  it. 

In  brief,  the  feet  should  be  dry  and  neither  distinctly 
warm  nor  cold ;  and  anything  which  interferes  with  these 
conditions  should  be  attended  to.  Shoes  and  stockings 
should  be  changed  as  frequently  as  necessary  and  only 
such  foot  wear  used  as  maintains  as  far  as  possible  the 
ideal  conditions  given  above. 

7.  The  Proper  Physical  Training  of  the  Foot.  —  It  is 
quite  possible  to  meet  all  the   above  hygienic  conditions 
and  yet  have  feet  which  are  incapable  of  doing  the  work 
which  we  have  a  right  to  demand  of  them.    As  was  shown 
at  the  outset,  the  action  of  the  foot  in  bearing  the  weight 


HYGIENE  OF  THE  FEET  411 

of  the  body  is  not  a  passive  but  an  active  one.  Muscles 
must  assist  in  holding  the  bones  in  place  when  one  is 
standing  still,  and  they  must  operate  the  foot  during  the 
act  of  locomotion.  The  physical  training  of  the  foot,  there- 
fore, consists  (1)  in  securing  adequate  strength  of  these 
muscles,  and  (2)  in  establishing  right  habits  in  using  them. 

Since  the  muscles  in  which  strength  is  especially  needed 
are  those  which  produce  plantar  (downward)  flexion  of  the 
toes,  we  may  strengthen  these  muscles  by  such  exercises 
as  pressing  as  hard  as  possible  with  the  toes  against  the 
floor  or  the  footboard  of  a  bed,  by  attempting  to  "  stand 
on  tiptoe,"  and  by  the  familiar  gymnastic  movement  of 
"  heels  raise,  knees  bend,"  etc. 

Among  the  habits  which  should  be  cultivated  may  be 
mentioned,  first  of  all,  walking  and  running  with  the  foot 
straight  forward  instead  of  toeing  outward.  The  bones 
which  form  any  hinge  joint,  like  that  of  the  ankle,  should 
move  in  a  plane  perpendicular  to  the  axis  of  motion  in  the 
joint,  and  this  is  possible  in  the  case  in  question  only  when 
the  feet  are  pointed  forward.  It  is  absolutely  wrong  to 
teach  children  to  toe  outward  in  walking,  and  they  would 
never  do  so  were  they  left  to  themselves  and  their  feet 
clothed  in  proper  shoes. 

In  addition  to  this,  the  habit  should  be  cultivated  of 
completing  each  step  by  "  digging  into  the  ground  "  with 
all  the  toes.  This  cultivates  the  use  of  the  foot  muscles 
in  locomotion  along  with  the  use  of  those  which  raise  the 
heel,  and  the  habit  once  acquired  and  regularly  practiced 
keeps  these  muscles  strong. 

Finally,  it  must  be  remembered  that  the  training  of  these 
muscles,  like  the  training  of  all  others,  must  be  a  gradual 
process.  Where  they  have  been  weakened  by  improper 
use,  one  must  proceed  to  strengthen  them  little  by  little 
from  day  to  day,  and  in  no  case  make  the  mistake  of 
imposing  upon  them  work  which  they  are  unable  to  bear. 


412  THE  HUMAN  MECHANISM 

Most  cases  of  "  weak  ankles  "  can  be  cured  if  taken  in 
time  and  their  muscles  gradually  trained.  But  these  mus- 
cles can  never  be  trained  by  imposing  upon  them  sudden 
and  severe  work  which,  in  their  weakened  condition,  they 
are  unable  to  perform.  The  fatigue  thus  induced  too  often 
prevents  their  working  at  all,  thus  leaving  the  weight  of 
the  body  free  to  strain  ligaments  and  do  other  damage 
which  may  leave  the  foot  in  a  worse  condition  than  before. 

We  have  already  insisted  (p.  319)  upon  the  importance 
of  walking  as  a  means  of  general  muscular  activity ;  and 
we  may  urge  in  concluding  this  chapter  that  the  chief 
hygienic  importance  of  the  care  and  training  of  the  feet 
lies,  not  so  much  in  the  fact  that  the  danger  of  acquiring 
flat  foot  is  thereby  lessened,  as  in  the  fact  that  we  thereby 
maintain  in  good  working  order  this  essential  part  of  the 
mechanism  of  locomotion.  American  men,  and  especially 
American  women,  compare  very  unfavorably  with  their 
English  cousins  in  the  ability  to  enjoy  walking  and  tramp- 
ing ;  and  while  this  is  partly  due  to  the  general  disuse  of 
walking  as  a  means  of  exercise,  it  is  equally  attributable 
to  the  deformation  of  the  feet,  which  robs  those  organs  of 
the  power  and  even  the  possibility  of  performing  with  ease 
their  proper  function. 


CHAPTER   XXIV 
BATHING 

1.  The  Hygiene  of  Bathing.  —  The  principal  hygienic 
purpose  of  bathing  may  be  stated  in  one  word,  namely, 
cleanliness.  A  bath  is  often  stimulating  and  refreshing, 
and  special  kinds  of  baths  may  be  used  upon  occasions  for 
good  and  useful  ends ;  their  value  in  the  treatment  of 
many  diseases  is  coming  to  be  widely  recognized,  and 
even  in  health  they  may  be  useful  as  aids  to  the  best  work- 
ing power.  But  experience  shows  that  it  is  not  necessary, 
even  if  it  be  refreshing,  for  a  healthy  person  leading  a 
healthy  life  to  use  bathing  for  any  other  purpose  than 
cleanliness. 

The  sweat  glands  and  the  sebaceous  glands  pour  out 
upon  the  skin  secretions  which  primarily  serve  the  useful 
purposes  of  regulating  the  temperature  of  the  body  and 
keeping  pliable  the  horny  layer  of  the  epidermis.  Each  of 
these  secretions  contains  solid  material,  which,  as  the  water 
of  perspiration  evaporates,  is  left  on  the  surface  of  the 
skin  or  in  the  ducts  of  the  sweat  glands  ;  some  of  the  solids, 
too,  are  either  themselves  odorous  or  else  are  putrescible, 
giving  rise  to  offensive  decomposition  products ;  conse- 
quently it  is  a  duty  which  every  one  owes  to  his  fellow-man 
to  bathe  so  as  to  be  clean  and  to  render  that  bathing  effective 
by  wearing  clean  clothes.  A  clean  person,  clean  clothing,  a 
clean  house,  clean  premises,  clean  streets,  a  clean  town  are 
so  many  forms  of  that  habit  of  cleanliness  which  is  one  of 
the  characteristics  of  high  civilization,  one  of  the  funda- 
mental elements  of  self-respect  and  proper  living. 

413 


414  THE  HUMAN  MECHANISM 

Besides  this,  filth  and  dirt  are  effective  carriers  of  dis- 
ease ;  consequently  bathing  and  the  use  of  clean  clothing 
diminish  the  chance  of  infection.  Finally,  personal  clean- 
liness also  keeps  the  skin  in  a  healthy  condition,  and  this 
alone  is  a  sufficient  reason  for  making  it  a  rule  in  the 
hygienic  conduct  of  life. 

2.  The  Indifferent  Bath.  —  A  bath  which  is  neither  dis- 
tinctly cold  nor  hot  may  be  said,  in  general,  to  answer  all 
purposes  of  cleanliness.    The  temperature  of  such  a  bath 
varies  between  80°  and  90°  F.  with  different  individuals. 
When  soap  is  used,  water  of  this  temperature  removes  the 
waste  products  from  the  skin  sufficiently  for  all  practi- 
cal purposes,  especially  when  such  a  bath  is  taken  daily. 
Indifferent  baths  are,  however,  without  any  stimulating 
(or  depressing)  physiological  effect,  provided  they  are  not 
taken  in  a  cold  room  ;  and  for  some  people  they  are  the 
most  advisable  form  of  bathing. 

3.  The  Hot  Bath  used  alone  is  not  as  a  rule  advisable. 
It  has  a  well:recognized  enervating  effect,  and  experience 
shows  that  after  it  great  caution  is  required  as  to  exposure 
to  cold.    These  effects,  however,  are  generally  obviated  by 
following  the   hot  bath  with  a  cold  needle  bath,  a  cold 
shower,  or  a  cold  plunge,  and  possibly  this  procedure  may 
be  recommended  as  the  most  useful  and  beneficial  form  of 
bathing  for  the  great  majority  of  people.    The  hot  bath 
serves  the  purposes  of  cleanliness  more  effectively  than 
the  indifferent  bath,  and  the  shock  of  the  cold  bath  is  not 
so  trying  to  many  people  when  taken  immediately  after 
the  skin  has  thus  been  thoroughly  warmed.    Too  frequent 
and  especially  too  prolonged  hot  bathing,  however,  is  apt 
to  remove  too  much  oil  from  the  skin. 

Hot  baths,  either  of  the  body  as  a  whole  or  at  times  a 
hot  footbath,  are  often  useful  in  bringing  the  blood  to  the 
skin  and  thus  checking  a  threatened  cold  or  other  inflam- 
matory process.  Special  care  is  needed,  however,  in  this 


BATHING  415 

case  to  avoid  subsequent  exposure  to  cold.  It  should  also 
be  remembered  that  a  very  hot  bath  is  a  strong  stimulus  to 
the  nervous  system  as  a  whole. 

4.  The  Cold  Bath  is  a  powerful  stimulus  to  the  nervous 
system.  When  the  irritability  of  the  latter  is  low,  as  when 
we  awake  from  slumber,  it  "  wakes  us  up,"  and  immedi- 
ately after  it  we  feel  distinct  exhilarating  effects.  In 
addition  to  this  it  probably  serves  as  a  training  to  the 
heat-regulating  mechanism  of  the  body,  "  hardening  "  the 
body  to  the  effects  of  cold.  Undoubtedly  its  influence 
with  a  large  proportion  of  healthy  people  is  beneficial, 
though,  as  we  shall  see,  this  is  not  the  case  with  all. 
Before  dealing  with  this  side  of  the  question  we  may  give 
some  rules  which  are  always  applicable  in  the  use  of  such 
baths. 

First,  they  should  not  be  prolonged.  To  stay  in  a  cold 
bath  longer  than  one  minute  is  undesirable  save  in  a  very 
few  exceptional  cases ;  thirty  seconds  is  the  usual  time, 
while  with  some  people  ten  seconds  is  the  maximum. 

Second,  a  cold  bath  should  be  taken  when  the  skin  is 
warm.  Immediately  on  rising  in  the  morning,  immediately 
after  muscular  exercise,  or  immediately  after  a  hot  bath  it 
is  most  beneficial  and  least  likely  to  produce  bad  after 
effects. 

Cold  bathing  should  always  be  followed,  except  in  warm 
weather,  by  a  good  rub-down  with  a  rough  towel.  This 
promotes  a  good  flow  of  blood  through  the  skin  and  adds 
to  the  tonic  effects. 

A  cold  bath  should  not  be  taken  in  a  cold  room.  Many 
profit  by  its  use  in  summer,  but  experience  undesirable 
effects  in  winter. 

Third,  cold  bathing  should  not  be  used  unless  it  is  fol- 
lowed by  what  is  called  the  "reaction";  that  is,  unless  it 
produces  a  distinct  glow  in  the  skin.  The  persistence  of 
pallor  in  the  skin  after  the  rub-down  is  proof  that  the 


416  THE  HUMAN  MECHANISM 

system  does  not  react  properly,  and  is  a  warning  that  this 
form  of  bathing  should  be  given  up  or  at  least  modified. 
This  does  not  mean  that  the  bath  necessarily  agrees  with 
us  if  it  does  produce  the  "glow,"  for  this  is  only  one  of  its 
after  effects,  and  we  must  judge  of  its  usefulness  not  by  one 
but  by  the  sum  total  of  the  effects  produced. 

Fourth,  no  bath,  unless  it  be  possibly  the  indifferent 
bath,  should  be  taken  within  an  hour  or  more  after  a  meal. 
The  evidence  of  experience  on  this  point  is  so  unmistakable 
that  nothing  more  need  be  said  about  it. 

It  would  be  a  mistake  to  discourage  all  bathing  except 
that  which  is  used  for  purposes  of  cleanliness ;  and  when 
we  insist  that  both  hot  and  cold  baths  are  an  artificial  ele- 
ment introduced  into  the  environment,  it  is  only  to  enforce 
the  need  of  carefully  observing  the  effects  of  their  use. 
No  one  is  justified  in  saying  that  these  baths  are  neces- 
sarily good  for  all  healthy  people ;  no  one  is  justified  in 
recommending  them  as  essential  elements  in  the  hygienic 
conduct  of  life.  They  must  be  judged  by  their  effects,  and 
when  submitted  to  this  standard  it  would  appear  that 
while  they  are  beneficial  to  some  people  they  are  harmful 
to  others. 

We  must  furthermore  distinguish  between  the  immediate 
results,  those  noticed  later  in  the  day,  and  the  remote 
results.  The  immediate  effects  may  be  exhilarating;  we 
may  "feel  splendid"  afterward,  and  yet  this  feeling  may 
be  succeeded  by  one  of  depression.  At  times  cold  bathing 
on  rising  in  the  morning  results  in  constipation,  although 
the  bath  itself  may  be  enjoyable.  This  may  be  exceptional, 
but  it  shows  that  every  one  must  determine  for  himself 
the  value  of  the  bath  by  the  sum  total  of  its  after  effects, 
and  not  alone  by  those  which  accompany  or  immediately 
follow  it. 

5.  Swimming  and  Salt-Water  Bathing.  —  When  one  is 
swimming,  the  heat  produced  within  the  body  by  muscular 


BATHING  417 

activity  counteracts  to  some  extent  the  effect  of  the  cool 
or  cold  water  applied  to  the  skin.  Hence  it  is  possible 
to  remain  in  the  water  a  longer  time  with  safety  and 
even  with  profit  than  in  the  ordinary  cold  bath.  It  is 
quite  impossible,  however,  to  give  definite  rules  as  to  the 
length  of  time  one  should  remain  in  the  water,  since  this 
depends  on  the  amount  of  muscular  activity,  on  the  tem- 
perature of  the  water,  and  on  the  condition  of  the  bather. 
But  the  hygienic  value  of  swimming  and  sea  bathing  must 
be  determined  by  the  same  tests  as  have  been  urged  in  the 
case  of  cold  bathing  in  general. 

It  is  also  important  to  remember  the  danger  of  going 
into  cold  water  when  one  is  fatigued  from  muscular  activ- 
ity. The  fatigued  muscles  seem  especially  liable  to  go 
into  cramps  under  these  conditions,  and  persons  have  been 
drowned  in  this  way  before  help  could  reach  them. 

When  one  takes  vigorous  daily  exercise  the  best  time 
for  the  bath  is  immediately  after  the  exercise.  One  is  then 
in  a  perspiration  and  it  is  best  to  change  the  clothing. 
The  skin  is  most  readily  cleaned  in  this  condition,  and 
most  persons  find  a  hot  bath,  with  or  without  the  use  of 
soap,  followed  by  a  short,  cold  needle  bath,  shower,  or 
plunge,  preferable  to  other  forms  of  bathing.  The  time  for 
bathing,  however,  like  the  time  for  eating,  must  depend 
on  one's  work  in  life.  We  do  not  live  to  bathe,  any  more 
than  we  live  to  eat. 


CHAPTER  XXV 
CLOTHING1 

1.  The  Hygienic  Object  of  Clothing.  —  Even  in  the  sav- 
age state  some  races  clothe  themselves  thoroughly.  The 
Eskimos,  for  example,  go  warmly  clad  in  furs,  and  the 
wild  Indians  who  once  inhabited  the  northern  United 
States  wore,  at  least  in  winter,  the  skins  of  animals.  In 
the  tropics,  on  the  other  hand,  as  in  northern  Africa  or 
the  islands  of  the  South  Seas,  very  little  clothing  is  worn, 
and  that  more  for  the  sake  of  decency  or  ornament  than 
for  warmth.  In  these  facts  we  find  the  hygienic  reason 
for  the  use  of  clothing,  namely,  to  aid  the  body  in  maintain- 
ing its  constant  temperature.  In  cold  weather,  clothing  is 
a  kind  of  portable  house,  a  close  and  intimate  shelter,  an 
indispensable  aid  to  the  skin  in  preventing  undue  loss  of 
heat ;  on  the  other  hand,  summer  clothing  should  interfere 
no  more  than  is  unavoidably  necessary  with  the  dissipation 
of  heat  from  the  skin.  If,  in  winter,  warm  days  come,  or 
if  the  body  becomes  heated  by  muscular  activity,  or  if  (as 
too  often  happens)  houses  or  public  places  are  overheated, 
then  winter  clothing  may  not  only  become  a  burden  but 
may  be  actually  unhygienic.  Conversely,  if  in  a  change- 
able climate  cold  days  or  nights  come  in  summer,  or  sea 
winds  blow  damp  as  well  as  cold,  then  ordinary  summer 
clothing  may  prove  to  be  insufficient.  Here,  as  always, 
the  individual  must  be  the  watchful  guardian  of  his  own 
welfare. 

1  The  student  is  advised  to  review  Part  I,  Chapter  XII,  before  studying 
this  chapter. 

418 


CLOTHING  419 

Clothing  affects  the  temperature  regulation  of  the  body 
chiefly  through  its  influence  upon  three  processes  by  which 
heat  is  taken  from  the  skin.  These  processes  are  (1)  con- 
vection by  wind  or  other  currents  of  air,  (2)  conduction, 
and  (3)  the  evaporation  of  perspiration.  We  shall  deal  with 
each  of  these  in  some  detail. 

1.  Clothing  and  the    Convection  of  Heat.  —  Any  fabric 
whose  texture  permits  the  air  warmed  by  contact  with  the 
skin  to  be  replaced  readily  by  colder  air  from  without  will 
obviously  favor  the  cooling  of  the  skin  by  convection ;  and 
conversely,  any  garment  which  lessens  or  altogether  pre- 
vents these  currents  of  air  through  it  is  to  that  extent  a 
warm  garment.    The   leather  hunting  jacket  lined  with 
wool  or  fur  is  especially  warm,  and  a  newspaper  under 
one's  coat  or  jacket  similarly  affords  a  large  measure  of 
protection  against  cold.    On  the  other  hand,  a  rubber  coat 
may  be  very  uncomfortable  on  a  warm  day,  although  the 
effect  in  this  case  is  due  to  its  interference  with  the  evap- 
oration of  the  perspiration  as  well  as  to  the  prevention 
of  the  passage  of  air  through  the  garment. 

2.  Clothing  and  the  Conduction  of  Heat. — Even  when  there 
is  no  passage  of  air  through  the  clothing,  heat  may,  of  course, 
be  transferred  from  the  skin  to  the  outer  air  by  conduction, 
and  some  fabrics  conduct  heat  more  readily  than  others. 
Other  things  being   equal,  the  rate  at  which  clothing  con- 
ducts heat  depends  on  the  amount  of  air  within  its  meshes. 
Thus  wool  is  warmer  than  cotton,  not  because  of  any  differ- 
ence in  conductivity  of  the  two  kinds  of  fibers,  but  because 
when  wool  fibers  are  made  into  yarn  their  stiffness  and 
elasticity  keep  them  apart,  so  that  garments  woven  from 
this  yarn  always  contain  spaces  filled  with  air,  which  is  a 
poor  conductor  of  heat.    Moreover,  the  same  properties  of 
the  fibers  prevent  their  being  pressed  and  felted  together 
in  laundering,  as  ordinarily  happens  with  cotton  and  linen 
fabrics.    We  shall  see  that  cotton  and  linen  may  be  so 


420  THE  HUMAN  MECHANISM 

woven  as  to  avoid  this  result,  as  in  many  "  mesh  work" 
fabrics,  but  they  are  not  usually  so  woven. 

A  moment's  thought  will  show  that  the  warmth  of  a 
dry  garment  will  depend  on  the  size  of  its  meshes.  These 
may  be  so  fine  and  close  as  to  inclose  an  insufficient  quan- 
tity of  the  non-conducting  air,  or  they  may  be  so  large  as 
to  permit  too  free  circulation.  In  the  latter  case  heat  is 
carried  away  from  the  skin  by  convection.  It  is  also  clear 
that  the  warmth  of  a  garment  is  not  determined  by  its 
weight  or  thickness  alone. 

3.  Clothing  and  the  Perspiration.  —  So  long  as  the  meshes 
of  a  fabric  contain  air,  heat  is  conducted  but  slowly  from 
the  skin.  When,  however,  this  air  is  partially  or  entirely 
replaced  by  water,  the  fabric  conducts  heat  from  the  skin 
much  more  rapidly;  and  if  the  surrounding  atmosphere 
is  distinctly  colder  than  the  body,  the  skin  becomes  chilled 
and  internal  organs  congested;  hence  the  danger  of  wet 
clothing. 

More  important  still  is  the  relation  of  clothing  to  the 
evaporation  of  perspiration.  We  have  learned  that  per- 
spiration is  useful  to  the  body  only  as  it  evaporates.  Con- 
sequently the  clothing  should  be  such  as  will  permit  the 
perspiration  to  evaporate  almost  as  fast  as  it  is  secreted. 
The  skin  will  thus  be  cooled  at  the  time  that  the  needs 
of  the  body  require  such  cooling,  and  the  clothing  will  not 
remain  wet  after  the  secretion  of  perspiration  has  ceased 
and  the  need  for  cooling  the  skin  no  longer  exists.  Or, 
if  it  is  not  possible  to  secure  this  rapid  drying,  the  fabric 
should  contain,  even  while  moist,  a  considerable  quantity 
of  air  within  its  meshes,  thereby  checking  the  loss  of  heat 
from  the  skin. 

2.  The  Clothing  worn  next  the  Skin  and  the  Outer  Cloth- 
ing. —  Consideration  of  the  above  relations  of  clothing 
to  convection,  conduction,  and  the  evaporation  of  perspi- 
ration shows  at  once  that  the  clothing  worn  next  the  skin 


CLOTHING  421 

must  fulfill  requirements  not  demanded  of  the  outer  cloth- 
ing. The  sole  hygienic  purpose  of  the  latter  is  warmth 
and  the  fabric  should  be  chosen  accordingly.  In  warm 
weather,  in  well-heated  rooms,  and  during  muscular  activ- 
ity, warm  outer  clothing  is  undesirable ;  on  the  other  hand, 
when  the  body  is  exposed  to  cold  and  is  not  at  the  same 
time  engaged  in  muscular  exertion,  the  outer  clothing 
should  be  chosen  for  warmth ;  and  for  this  purpose  woolen 
fabrics  are  superior  to  all  others. 

The  clothing  worn  next  the  skin  must,  in  addition,  care 
for  the  perspiration.  For  those  forced  by  age  or  other 
physical  disability  to  lead  sedentary  lives,  woolen  under- 
wear is  very  useful  in  cold  weather.  Since  in  the  case 
of  such  persons  the  blood  is  not  brought  to  the  skin  by 
muscular  activity,  it  is  necessary  that  the  skin  be  kept 
warm  and  internal  congestions  prevented.  For  such  per- 
sons woolen  fabrics  are  probably  superior  to  all  others. 
Moreover,  during  exposure  to  extreme  cold,  when  little  or 
no  perspiration  is  secreted  even  during  vigorous  muscular 
work,  woolen  underwear  is  superior  for  every  one  because 
of  its  greater  warmth. 

For  healthy  people,  however,  in  the  full  vigor  of  life, 
taking  daily  muscular  exercise  but  not  exposed  to  extremes 
of  cold,  woolen  underwear  presents  many  serious  draw- 
backs. In  the  first  place  its  very  warmth  is  objectionable 
during  muscular  activity,  because  it  makes  more  difficult 
the  discharge  of  the  surplus  heat.  In  the  second  place, 
wool  absorbs  the  perspiration  very  slowly  and  so  prevents 
its  evaporation  from  the  outer  surface  of  the  garment ; 
the  perspiration  does  not  cool  the  body  as  it  should,  but 
remains  between  the  skin  and  the  garment,  —  an  unhealth- 
ful  condition  for  the  skin.  In  the  third  place,  when  the 
garment  has  once  become  "  wet  through,"  i.e.  the  air 
within  its  meshes  has  been  largely  displaced  by  water,  it 
dries  more  slowly  than  a  linen  or  a  cotton  garment. 


422  THE  HUMAN  MECHANISM 

It  is  better,  in  other  words,  for  healthy  people  to  depend 
upon  the  outer  clothing,  including  overcoats,  etc.,  for 
warmth,  when  protection  against  cold  is  needed,  rather 
than  upon  even  moderately  heavy  underwear.  In  this  way 
it  is  possible  readily  to  relieve  the  body  of  its  heavier 
clothing  when  it  becomes  necessary  to  get  rid  of  surplus 
heat,  i.e.  in  warm  rooms  and  during  muscular  activity  in 
only  moderate  cold  weather,  and  yet  to  protect  oneself 
against  cold  when  such  protection  is  necessary. 

Of  late  years  the  attempt  has  been  made  with  consid- 
erable success  to  weave  linen,  and  even  cotton,  so  as  to 
contain  fairly  large  meshes  between  the  threads.  The 
perspiration  is  rapidly  brought  to  the  surface  of  the  gar- 
ment through  the  threads  by  capillary  attraction  and  so 
evaporates  quickly;  for  this  reason  the  garment  dries 
readily,  and  even  while  wet  usually  retains  a  considerable 
quantity  of  air  within  its  meshes. 

The  thickness  of  underwear,  as  well  as  of  the  garments 
worn  immediately  over  it,  should  be  determined  by  the 
amount  of  exposure  to  cold  when  at  rest.  When  our  houses 
or  offices  are  properly  heated  (65°-70°  F.)  in  winter,  heavy 
clothing  is  as  much  to  be  condemned  as  the  too  common 
overheating  of  our  rooms,  and  for  the  same  reason.  When, 
on  the  other  hand,  our  work  is  out  of  doors  in  cold  weather 
but  involves  only  a  small  amount  of  muscular  activity, 
warmer  clothing  should  be  worn ;  in  this  case  the  use  of 
heavy  woolen  underwear  is  advisable. 

It  is  unnecessary  to  go  further  into  details.  The  stu- 
dent can  solve  special  problems  for  himself,  always  remem- 
bering that  proper  clothes  are  such  as  will  prevent  undue 
loss  of  heat  and  consequent  chilling  of  the  skin  (with 
accompanying  internal  congestions)  when  the  body  is  at 
rest. 

3.  The  Outer  Clothing.  —  Of  this  little  need  be  said.  By 
varying  the  thickness  of  the  outer  clothing  we  adapt  it 


CLOTHING  423 

to  the  conditions  of  life.  It  must  also  be  chosen  with  ref- 
erence to  its  permeability  to  air.  In  hot  summer  weather 
it  should  be  as  thin  and  porous  as  possible ;  in  winter  it 
should  protect  from  wind.  When  still  further  protection 
is  needed,  it  may  be  obtained  by  the  use  of  overcoats, 
gloves,  muffs,  lap  robes,  or  other  wraps. 

Some  people  do  not  use  sufficiently  warm  clothing  in 
cold  weather,  but  most  adults  make  the  opposite  mistake. 
The  custom  of  using  very  thick  clothing  in  cold  weather 
appears  to  have  been  inherited  from  the  time  when  houses 
were  poorly  heated,  when  transportation  from  place  to 
place  was  in  cold  cars  or  carriages,  and  when,  in  general, 
the  human  race  was  more  exposed  to  cold  than  it  is  to-day. 
Where  these  conditions  prevail,  as  they  still  do  in  many 
country  districts,  heavy  clothing  should  no  doubt  be  worn 
in  winter.  The  same  may  be  said  of  driving  in  open  vehi- 
cles, such  as  sleighs,  etc.  But  in  cities,  where  houses  are 
more  likely  to  be  overheated  than  underheated,  where  steam 
and  electric  cars  are  far  from  being  chilly,  where,  in  short, 
we  need  not  generally  be  exposed  to  cold  except  when 
walking  or  taking  other  muscular  exercise,  the  main 
dependence  for  protection  against  cold  should  be  upon 
the  outer  wrappings  rather  than  upon  the  underwear,  the 
coat  and  trousers,  or  the  dress.  We  do  not  change  to 
heavy  clothing  in  summer  when  the  thermometer  falls  to 
65°  or  70°  F.,  and  there  is  no  reason  why  we  should  use 
such  clothing  at  these  temperatures  in  winter.  The  pre- 
cautions which  many  take  against  sudden  changes  of 
weather  are  often  excessive. 

4.  Clothing  not  the  only  Protection  against  Cold.  —  It 
must  be  remembered  that  we  have  another  means  of  pro- 
tection against  cold  besides  clothing,  and  that  is  muscular 
activity.  Even  if,  as  often  happens,  a  balmy  morning  passes 
into  a  chilly  afternoon,  most  people,  especially  those  living 
in  cities,  should  be  able  to  keep  warm  by  a  brisk  walk 


424  THE  HUMAN  MECHANISM 

when  going  home  ;  a  little  exposure  to  cold  will  not  harm, 
but  will  rather  harden,  a  healthy  man  or  woman.  If  we  are 
tired  out  and  ought  not  to  walk,  we  can  usually  ride  in 
a  heated  car.  To  wear  heavier  clothing  than  the  probable 
necessities  of  the  case  demand,  merely  because  there  is  a 
chance  that  suitable  weather  for  such  clothing  may  over- 
take us,  is  in  general  unwise.  Oppressed  with  its  weight 
and  warmth,  the  usual  result  is  a  disinclination  to  any 
vigorous  muscular  activity  while  out  of  doors,  and  this  in 
the  long  run  is  more  dangerous  than  a  comparatively  brief 
chilling  of  the  skin. 

5.  Clothing  should  not  be  Heavy.  —  The  reference  in  the 
last  paragraph  to  the  burden  of  heavy  clothing  deserves 
further  consideration.  The  terms  "warm,"  "thick,"  and 
"heavy,"  as  applied  to  clothing,  are  often  used  as  if  they 
were  synonymous,  although  a  thick  garment  is  not  neces- 
sarily a  heavy  garment,  and  a  thinner  but  more  loosely 
woven  coat  may  be  warmer  than  orie  which  is  thicker  but 
more  closely  woven.  In  the  selection  of  clothing  it  is 
always  advisable,  not  only  as  a  matter  of  personal  comfort 
but  also  as  a  matter  of  practical  hygiene,  to  avoid  heavy 
fabrics.  While  this  holds  especially  for  invalids  and  elderly 
people,  to  whom  the  burden  is  more  oppressive,  it  also 
holds  for  the  young  and  strong.  The  clothing  should  be 
such  as  will  interfere  in  the  least  degree  with  the  freedom 
of  bodily  movements.  Not  only  should  every  one  avoid 
such  fashions  as  tight  lacing  and  high-heeled  boots,  —  so 
senseless  as  to  be  beneath  the  contempt  of  those  who 
respect  the  human  body  and  care  for  its  physical  well- 
being, —  but  care  should  be  taken  to  have  the  clothing 
everywhere  loose  enough  to  be  comfortable  and,  above 
all,  light  enough  so  that  its  weight  is  not  a  burden.  For 
this  reason  a  very  close  weave  is  objectionable  except  in 
windy  weather,  since  it  gives  great  weight  of  fabric  with 
but  small  air  contents. 


DOMESTIC  HYGIENE  AND  SANITATION 


CHAPTER  XXVI 

THE    HOUSE:    ITS    SITE,    CONSTRUCTION,    FURNISHINGS, 
AND  CARE 

1.  The  Family  a  Private  Community.  —  Every  human 
being  has  not  only  individual  or  personal  relations  with  his 
environment,  but  also  various  other,  and  public  relations, 
since  the  life  of  an  individual  is  always  more  or  less  closely 
connected  with  the  lives  of  other  human  beings.  Each 
individual  or  person  is  a  member  of  some  family,  and  also 
of  some  village,  town,  city,  state,  or  nation.  Connections 
of  this  kind  constitute  kinship,  relationship,  and  fellow- 
ship, and  are  commonly  described  as  social  relations  (socius, 
a  fellow).  They  are  nowhere  more  conspicuous  than  in 
matters  of  life  and  death,  health  and^disease.  The  human 
infant  is  absolutely  dependent  upon  parental  care,  and 
among  civilized  people  the  sick,  the  aged,  the  dying,  and 
the  dead  must  be  tenderly  cared  for  by  those  who  are  alive 
and  well.  But  this  is  not  all,  for  sickness  is  frequently 
"  catching,"  and  plagues,  pestilences,  and  epidemics  have 
often  run  like  wildfire  through  families  or  communities, 
leaping  from  person  to  person,  and  from  village  to  village, 
very  much  as  a  forest  fire  leaps  from  tree  to  tree. 

A  fundamental  feature  of  all  social  relations  is  the  fact 
that  persons  in  families,  villages,  towns,  cities,  states, 
and  nations  have  and  use  many  things  in  common.  This 
has  caused  such  groups  of  human  beings  to  be  known  as 

425 


426  THE  HUMAN  MECHANISM 

communities  (communis,  common).  Of  all  communities  the 
simplest,  the  most  fundamental,  and  the  most  important  is 
the  family  or  household,  in  which  the  various  individual 
members  share  a  common  shelter,  a  common  fireside,  a 
common  table,  and  a  common  interest,  based  upon  the  all- 
powerful  ties  of  blood  or  marriage.  In  these  and  many 
other  respects  the  family  is  not  only  a  community  but  a 
peculiar  kind  of  community,  namely,  a  private  community. 
But  precisely  as  the  individual  necessarily  has  relations 
to  the  world  outside  himself  and  is  by  nature  not  merely  a 
man  and  an  animal,  but  a  social  man  and  a  social  animal, 
so  the  civilized  family  or  household,  although  essentially  a 
private  establishment,  has  certain  public  relations.  It  must 
draw  its  air  supply  from  the  aerial  ocean  common  to  all 
mankind ;  it  must  form  a  component  unit  in  some  village, 
township,  state,  or  nation ;  it  must  buy  sugar  or  salt,  or 
tea,  coffee,  or  spices,  from  overseas. 

Midway  between  the  more  purely  public  relations  which 
we  shall  presently  discuss  under  public  hygiene  and  sani- 
tation, and  those  individual  relations  which  we  have  con- 
sidered in  the  foregoing  chapters  on  personal  hygiene,  stand 
the  hygiene  and  sanitation  of  the  house  and  the  family, 
subjects  neither  altogether  public  nor  altogether  personal. 
These  we  may  describe  as  domestic  hygiene  and  sanitation. 

2.  Housing  and  the  House.  —  The  chief  function  of 
clothing  (p.  418)  is  to  protect  the  body  from  cold  by  main- 
taining about  the  skin  fairly  constant  temperature  condi- 
tions, and  accordingly  clothing  is  of  least  importance  in 
the  tropics,  where  conditions  of  temperature  are  both  con- 
stant and  warm.  The  housing  problem  is  very  similar, 
for  the  principal  function  of  the  house  is  likewise  to  fur- 
nish for  the  body  a  favorable  environment,  and  especially 
a  fairly  constant  temperature ;  here  also  the  problem  is 
simplest  in  the  tropics.  The  house,  in  fact,  is  a  kind  of 
outer  clothing  or  protective  shell,  although  usually  designed 


SANITAKY  HOUSING  427 

not  for  a  single  individual  but  for  an  entire  family,  or,  as 
in  the  case  of  tenement  houses,  apartment  houses,  or  hotels, 
for  many  families,  or  for  the  public. 

Savages  and  the  lower  races  of  men  content  themselves 
with  caves,  trees,  and  other  ready-made  shelters,  or  with 
huts,  hovels,  wigwams,  and  similar  poor  and  primitive 
dwellings.  The  most  highly  civilized  races,  on  the  other 
hand,  build  carefully  planned  and  sometimes  elaborate 
houses,  furnished  with  various  devices,  simple  or  complex, 
for  heating,  lighting,  ventilation,  water  supply,  drainage, 
cookery,  and  other  necessaries  or  luxuries  of  a  comfort- 
able, as  well  as  a  uniform  and  favorable,  environment. 

Houses  may  be  separated  and  detached,  as  on  farms  or 
in  villages,  or  massed  in  groups  or  blocks,  as  in  towns  or 
cities ;  and  owing  to  the  fact  that  they  are  comparatively 
durable  and  costly,  most  people  live  in  dwellings  already 
built.  But  although  very  often  a  family  cannot  build  its 
dwelling,  but  must  take  to  some  extent  what  it  can  get,  it 
usually  has,  sooner  or  later,  some  choice ;  and  even  if  it 
has  not  much  choice,  it  may  modify  more  or  less  from 
time  to  time  the  domicile  which  it  must  occupy. 

3.  The  Site  of  the  House  is  often  determined  more  by 
necessity,  taste,  or  convenience  than  by  hygienic  considera- 
tions; but  in  general  it  may  be  said  that  a  human  dwell- 
ing should  be  so  situated  as  to  afford  good  air,  good  light, 
good  drainage,  and  good  neighbors.  If,  in  addition,  beauti- 
ful, charming,  or  attractive  surroundings  can  be  had,  these 
are  of  great  importance,  since  beauty  and  charm  often  have 
a  distinct  hygienic  value.  A  certain  seclusion  or  privacy 
is  also  to  be  desired,  for  a  quiet,  retired,  and  restful  home, 
removed  from  the  distractions  of  publicity,  is  soothing 
to  tired  nerves  as  well  as  conducive  to  normal  and  whole- 
some family  life.  On  the  other  hand,  extreme  isolation, 
such  as  is  sometimes  found  in  farmhouses,  often  produces 
a  morbid  feeling  of  loneliness. 


428  THE  HUMAN  MECHANISM 

When  possible  the  house  should  be  placed  upon  open, 
porous,  or  gravelly  soil,  because  such  soil  is  less  likely  to 
be  water-logged  and  is  more  easily  drained.  In  the  United 
States  in  general  a  southerly  or  southwesterly  slope  is  usually 
preferable,  as  affording  more  sunshine  in  winter  and  more 
breeze  in  summer.  It  is  also  wise,  of  course,  to  have  the 
principal  living  rooms  on  the  side  exposed  to  dry  rather 
than  to  cold  and  damp  winds. 

G-ood  air  for  a  house  is  to  be  sought  for  in  a  clean  neigh- 
borhood, a  clean,  dry  cellar,  and  a  free  circulation,  the 
latter  impeded  as  little  as  possible  by  other  buildings  or, 
in  the  country,  by  too  many  trees.  An  elevation,  therefore, 
rather  than  a  depression  is  obviously  desirable  as  an  aid  in 
securing  these  things,  although  the  very  top  of  a  hill  should 
usually  be  avoided  because  of  its  bleakness.  A  dry  cellar 
and,  if  possible,  a  dry,  open,  and  porous  soil  beneath  the 
house  are  highly  important,  no  matter  where  the  house  is 
placed.  Cellar  habitations  are  very  objectionable  and  ought 
to  be  avoided  by  even  the  poorest  family.  Such  dwellings 
were  long  since  forbidden  by  law  in  England,  and  although 
sanitarians  are  not  yet  agreed  as  to  all  the  reasons  why  cellar 
habitations  are  injurious,  the  principal  reasons  seem  to  be 
the  well-known  unwholesomeness  of  dampness,  and  want  of 
sunlight.  Undue  dampness  in  air  is  believed  to  favor  rheu- 
matism and  other  disorders,  and  the  absence  of  sunlight  not 
only  favors  dampness  and  microbic  life  but  also  tends  to 
mental  depression.  House  cellars  should  be  well  drained. 

G-ood  light  and,  if  possible,  abundant  sunshine  are  hygienic 
conditions  of  great  importance,  both  as  aids  to  cheerful- 
ness and  happiness  and  as  powerful  sanitary  agents.  Sun- 
shine tends  to  remove  dampness  and  to  destroy  the  germs 
of  infectious  diseases.  In  winter,  sunshine  is  valuable  also 
for  warmth. 

G-ood  drainage  is  no  less  (and  perhaps  no  more)  necessary 
for  human  habitations  than  good  air  and  good  light.  With 


SANITARY  HOUSING  429 

'the  abundant  use  of  water  in  recent  times  for  washing, 
bathing,  cleaning,  sewage  disposal,  and  other  purposes,  it 
becomes  necessary  in  modern  houses  to  get  rid  somehow 
of  a  great  deal  of  soiled  and  dirty  water,  and  the  possi- 
bility of  easy  and  safe  "drainage,"  or  removal  of  such 
water,  must  be  kept  in  mind  in  considering  the  sanitary 
aspects  of  the  situation  of  any  house,  old  or  new.  Here 
again  the  advantage  is  plain  of  some  elevation  of  site. 

4.  The  Construction  of  the  House.  —  As  the  first  object 
of  any  house  is  shelter  from  rain,  snow,  wind,  dampness, 
and  excessive  heat  or  cold,  its  materials  should  be  water- 
proof, windproof,  and  non-conducting  for  heat,  as  far  as  is 
consistent  with  a  proper  circulation  of  air.  Wigwams  or 
tents,  at  least  in  temperate  latitudes,  are  clearly  defective 
in  some  of  these  particulars.  Houses  built  of  glass  or  india 
rubber  would  answer  most  of  these  requirements  but  would 
still  be  most  unhygienic,  chiefly  because  glass  houses  would 
be  too  light  and  too  hot,  while  both  glass  and  india  rubber 
would  interfere  seriously  with  that  free  circulation  of  air 
which  takes  place  through  relatively  porous  materials 
such  as  wood  and  brick.  Buildings  of  wood,  stone,  brick, 
or  steel  and  brick,  rightly  built,  answer  all  requirements. 
A  "  double  wall,"  i.e.  a  hollow  wall,  by  providing  a  non- 
conducting air  space  is  of  great  value  for  preventing 
rapid  changes  in  the  temperatures  of  houses  under  sudden 
changes  of  climate,  as  well  as  for  protection  against  damp- 
ness and  noise. 

Much  circulation  of  air  usually  takes  place  even  through 
walls  or  partitions  of  plaster  and  wood,  and  a  knowledge 
of  this  so-called  "  natural  ventilation  "  helps  us  to  under- 
stand how  it  is  that  many  people  live,  and  even  thrive,  in 
seemingly  unventilated  rooms  and  houses.  It  also  helps 
us  to  understand  how  the  damp  air  of  a  cellar  finds  its 
way  upward  into  a  house,  and  why  a  double  floor  (with 
air  spaces  between)  is  especially  useful  immediately  above 


430  THE  HUMAN  MECHANISM 

the  cellar.  Blinds  or  shutters,  and  shades  or  curtains,  for* 
darkening  rooms,  are  of  great  hygienic  value,  since  sleep 
is  deeper  in  darkness  than  in  light,  and  in  summer  these 
tend  also  to  keep  the  house  cooler. 

5.  The  Furnishings  of  the  House.  —  The  walls  of  rooms 
may  be  of  wood,  —  bare,  painted,  or  varnished,  —  or  of  plas- 
ter, —  either  bare  or  covered  by  textiles  such  as  burlap  or 
tapestry,  or  "papered"  by  pasting  upon  them  with  thick 
paste,  or  "  sizing,"  sheets  of  paper  upon  which  designs, 
often  in  color,  have  been  printed.  Sometimes,  instead  of 
being  papered,  walls  and  partitions  are  painted,  either  with 
white  paint  or  in  colors,  and  sometimes  simply  a  hard 
finish  is  given  to  plaster,  which  is  afterwards  "  white- 
washed "  or  "  calcimined." 

A  good  feature  of  painted  walls  is  the  fact  that  they 
may  be  washed,  and  of  walls  smoothly  calcimined  that 
they  may  be  easily  done  over.  In  general,  a  smooth  and 
washable  surface  is  preferable  to  a  rough  one  or  one  in- 
jured by  washing,  for  these  not  only  collect  more  dust  but 
are  harder  to  keep  clean. 

The  most  serious  charge,  from  the  hygienic  point  of 
view,  thus  far  brought  against  wall  papers,  is  that  of  the 
danger  of  poisoning  for  persons  living  in  rooms  papered 
with  such  papers  as  contain  arsenic.  The  subject  has 
been  much  in  dispute,  but  the  evidence  of  such  occasional 
poisoning  seems  now  convincing,  especially  since  the  work 
of  Gosio,  an  Italian  investigator,  showed  that  molds  or 
other  microorganisms  which  grow  in  the  paste  used  to 
stick  the  paper  to  the  walls  are  capable  of  attacking  the 
arsenic  of  some  coloring  matters,  thereby  producing  volatile 
compounds  of  arsenic  readily  diffusible  into  the  air  of  the 
room. 

There  is  similar  danger  from  arsenical  poisoning  in  some 
tapestries  or  furniture  coverings,  and  grave  disorders  have 
been  attributed,  apparently  with  reason,  to  this  source. 


SANITAEY  HOUSING  431 

The  iron  bedstead,  light,  firm,  cheap,  and  easy  to  keep 
clean,  is  a  marked  improvement  upon  the  heavy  wooden 
bedsteads  formerly  used.  Curtains,  canopies,  valances,  etc., 
either  above  or  below  beds,  are  objectionable,  as  they  inter- 
fere with  the  free  circulation  of  air.  The  modern  open 
bedstead  is  an  improvement  upon  the  old-fashioned  "  four- 
poster  "  with  its  hangings,  almost  as  great  as  is  the  modern 
"  open,"  over  the  earlier  concealed,  plumbing. 

Single  beds  possess  many  advantages  over  double  beds. 
They  are  more  easily  cared  for  and  kept  clean ;  the  amount 
of  covering  can  be  more  accurately  adapted  to  the  indi- 
vidual needs  of  their  occupants,  who  are  also  less  exposed 
in  cases  of  infectious  disease ;  and  the  use  of  such  beds  is 
more  conducive  to  undisturbed  slumber. 

Folding  beds,  mantle  beds,  sofa  beds,  and  all  similar 
devices  for  concealment  of  beds  and  bedding  are  subject  to 
the  objection  that  they  are  likely  to  be  closed  too  soon 
after  having  been  used,  and  before  the  bedding  has  been 
sufficiently  aired  or  freshened.  Certain  forms  of  folding 
beds  are  further  objectionable  from  the  fact  that  by  their 
closing  automatically  and  unexpectedly  the  occupant  has 
sometimes  been  imprisoned  and  even  killed. 

Floors  are  in  America  usually  wooden  and  made  of 
boards,  "  matched  "  or  otherwise  laid  tight.  Formerly  the 
material  used  for  inexpensive  floors  was  of  pine,  spruce, 
hemlock,  or  other  soft  woods,  oak  being  reserved  for  the 
more  costly  hard  floors.  Nowadays  hard  pine  (Southern 
pine)  is  much  used  and  many  cheap  yet  good  floors  are 
made  of  this  material.  Softwood  floors  are  apt  to  become 
dented  and  splintered  unless  covered  and  protected  by 
carpets  or  mattings ;  but,  if  made  of  good  stock  and  well 
cared  for  by  frequent  painting,  such  floors  answer  very 
well  for  a  long  time,  especially  in  rooms,  such  as  cham- 
bers, not  subject  to  hard  usage.  Bare  floors  possess  the 
immense  sanitary  advantage  of  being  easy  to  clean  and 


432  THE  HUMAN  MECHANISM 

also  of  revealing  dust  and  dirt.  But  they  require,  for  the 
latter  reason,  more  care,  and  are  also  open  to  the  objection 
that  they  are  comparatively  noisy. 

Fixed  mattings  are  useful  for  the  deadening  of  sounds, 
and  fixed  carpets  not  only  for  this  but  also  for  warmth  ;  but 
both  hold  dirt  and  are  hard  to  clean,  while  light,  movable 
mattings,  carpets,  or  rugs  readily  lend  themselves  to  cleanli- 
ness, because  they  can  be  removed  and,  in  their  temporary 
absence,  both  they  and  the  otherwise  bare  floors  upon  which 
they  rest  can  be  thoroughly  cleaned. 

6.  The  Care  of  the  House.  —  The  house,  the  outermost 
clothing  of  man  and  the  family,  like  the  dress  of  the  body 
or  the  clothing  proper,  is  subject  to  the  wear  and  tear  of 
time,  season,  and  weather,  and  likewise  requires  care,  con- 
sisting chiefly  of  cleaning  and  mending,  or  repair.  Paint- 
ing in  the  case  of  wooden  houses  (and  for  the  steel  parts 
of  steel-and-brick  structures),  and  pointing  (or  the  renewal 
of  mortar  between  bricks  or  stones)  in  the  case  of  brick  or 
stone  houses,  helps  to  make  them  waterproof  and  wind- 
proof  and  tends  to  keep  out  dampness. 

The  cellar,  especially,  requires  watchful  care,  and  should 
be  kept  not  only  dry,  by  windows  or  other  ventilating 
devices,  opened  wide  in  favorable  weather  (and  never 
wholly  shut),  but  also  clean  and  free  from  rubbish,  decay- 
ing vegetables,  or  anything  tending  to  dirt,  dampness,  or 
uncleanness. 

The  walls  of  the  rooms,  if  papered,  should  from  time  to 
time  be  wiped  with  damp  cloths,  or  if  painted,  they  may  be 
washed.  Floors  and  the  hard  parts  of  furniture  are  thor- 
oughly cleaned  only  by  washing,  but  may  also  be  well 
cared  for  by  wiping  with  damp  (not  wet)  cloths  which  are 
easily  washed  and  wrung  out.  Brushes  with  long  handles 
are  also  useful,  though  less  effective,  for  ceilings  and  bare 
floors,  and  brooms  for  fixed  mattings  and  carpets.  Dust 
cloths  and  the  little  feather  duster  may  be  used  upon  books, 


SANITAEY  HOUSING  433 

fabrics,  and  other  surfaces  which  would  be  injured  by  the 
damp  cloth;  but  such  "dusting,"  in  so  far  as  it  lifts  the 
dust  into  the  air  only  to  allow  it  to  settle  again  some- 
where in  the  same  room,  merely  transfers  it  from  one  place 
to  another,  and,  while  perhaps  better  than  nothing,  does 
not  produce  true  cleanness.  The  large  feather  duster  often 
used  to  clean  (?)  things  which  should  be  wiped  with  damp 
cloths,  easily  washed  or  renewed,  is  simply  a  sanitary 
abomination.  It  makes  a  show  of  cleanliness  but  lacks 
the  substance  thereof.  It  is  the  favorite  implement  of  the 
superficial,  indolent,  or  shiftless,  and  especially  of  janitors 
of  this  class  in  public  buildings. 

Even  when  closed,  houses  quickly  become  dusty  or  dirty, 
because  the  air  which  finds  its  way  into  them  through 
cracks  or  crevices  is  almost  always  more  or  less  charged 
with  dust,  while  the  occupants  of  inhabited  houses  bring 
in  upon  clothes,  shoes,  and  all  kinds  of  articles  more  or 
less  dust  or  dirt.  Fires,  such  as  those  in  stoves,  fireplaces, 
or  furnaces,  also  add  greatly  to  the  dust  of  houses.  Dust 
and  dirt  are  composed  largely  of  inorganic  or  lifeless  mat- 
ters such  as  particles  of  sand,  or  coal,  or  ashes,  or  fibers 
of  cotton  or  woolen,  but  also  partly  of  microbes.  Most  of 
the  latter  are  harmless,  and  some  kinds  of  dust  and  dirt 
are  of  little  sanitary  importance,  —  a  fact  which  helps  us 
to  understand  why  some  people  seem  to  have  health  even 
in  dirty  surroundings.  But  dust  and  dirt  sometimes  con- 
tain the  germs  of  dangerous  diseases,  and  the  way  of  safety 
is  the  way  of  cleanliness. 


CHAPTER  XXVII 
THE  WARMING  AND  LIGHTING  OF  THE  HOUSE 

1.  The  Warming  of  the  House.  —  The  earliest  method  of 
warming  human  dwellings  was  the  open  fire,  in  hut,  cave, 
or  wigwam,  and  when  chimneys  were  added  to  carry  off 
smoke   and  improve  combustion  by  creating  drafts,  the 
open  fire  still  remained  for  a  time  the  sole  resource  of 
mankind  for  heating  purposes.    It  is  still  the  most  attrac- 
tive and  most  cheerful  method  of  heating,  and  has  been 
well  called  "  the  eye  of  the  room."    It  is  a  coveted  luxury 
in  all  tasteful  homes,  not  so  much  for  the  heat  it  furnishes 
as  for  its  cheerful  glow  and  the  constant  interest  which  it 
excites.    The  home  and  the  fireside  have  become  everywhere 
almost  equivalent  terms. 

2.  The  Open  Fire  may  be  of  either  coal  or  wood.    In 
England  it  is  almost  always  of  coal,  and  in  that  country  is 
still  the  principal  means  of  heating.    In  some  other  coun- 
tries, and  especially  in  the  United  States,  it  is  made  of 
either  coal  or  wood,  but  is  less  depended  upon  for  heating 
purposes.    The  open  fire  may  be  on  a  hearth  in  a  fireplace, 
or  in  an  open  grate  or  an  open  stove.    In  all  modern  cases 
of  true  open  fires,  a  chimney  rises  above  the  fire  to  carry  off 
the  smoke,  and  the  draft  of  the  chimney  (caused  by  the  rising 
of  the  column  of  lighter  heated  air)  constantly  sucks  away 
the  air  of  the  room  and  produces  considerable  ventilation  by 
removing  vitiated  air.    The  air  thus  removed  is  replaced 
by  air  from  adjoining  rooms  or  from  outdoors,  driven  in 
by  the  atmospheric  pressure  through  open  doors  or  win- 
dows or  through  the  walls  themselves  which,  if  of  wood  or 

434 


WAKMING  AND  LIGHTING  OF  HOUSES      435 

plaster,  or  even  of  stone  or  brick,  are  to  some  extent 
porous.  But  while  such  ventilation  has  great  advantages 
and  is  one  of  the  best  things  about  open  fires,  such  fires 
are  wasteful  of  heat  and  often  do  not  effectively  warm  the 
entire  room.  This  is  because  the  warmed  air  is  not  returned 
to  the  room,  but  is  drawn  up  the  chimney,  and  because  the 
movement  of  the  cold  air  which  is  pressed  in  from  the 
outside  tends  to  make  the  room  "  draf ty."  Radiation  from 
the  fire  itself,  rather  than  convection  by  air  currents,  thus 
becomes  the  chief  means  of  warmth;  and  the  complaint 
against  open  fires  that  those  gathered  about  them,  whether 
indoors  or  out,  are  "  roasted  in  front  and  frozen  behind,"  is 
undoubtedly  well  founded.  Open  fires,  nevertheless,  serve 
admirably  to  "  take  the  chill  off "  from  a  room  in  those 
days  of  late  spring  or  early  autumn  when  the  temperature 
is  only  a  few  degrees  below  the  proper  point  (see  p.  201). 

3.  Stoves  are  superior  to  open  fires  as  sources  of  warmth, 
but  far  inferior  in  attractiveness  and  as  aids  to  ventila- 
tion.   A  stove  is  usually  placed  in  a  room  at  some  dis- 
tance from  the  wall,  and  connected  with  the  chimney  by  a 
stovepipe  to  carry  off  the  products  of  combustion.    There 
is  no  such  thing  as  an  "  air-tight "  stove,  a  term  often  used 
because  some  stoves  seem  tightly  closed,  only  enough  air 
being  allowed  to  enter  to  supply  the  actual  need  for  com- 
bustion.   A  stove  warms  a  room  by  the  mixture  of  currents 
of  heated  air  around  the  stove  with  the  cooler  air  in  other 
parts  of  the  room,  i.e.  by  convection,  and  also  by  direct 
radiation  from  the  fire  itself. 

4.  Hot-Air  Furnaces  are  usually  inclosed  stoves  placed 
in  the  basement  or  cellar.    They  are  provided  with  smoke 
pipes  and  surmounted  by  a  space,  the  hot-air  chamber,  to 
the  lower  portion  of  which  a  pipe,  or  "  air  box,"  conducts 
cold  air,  while  a  second  pipe  or  system  of  pipes  leading 
off  from  the  upper  portion  of  the  chamber  supplies  the 
various  rooms  with  the  warmed  air.    This  is  a  convenient, 


436  THE  HUMAN  MECHANISM 

economical,  and  popular  method  of  heating  a  house,  and 
possesses  the  great  advantage  of  bringing  constantly  into 
the  various  rooms  supplies  of  fresh  air.  If  this  air  has 
not  been  overheated  while  passing  by  the  furnace,  little 
objection  can  be  brought  against  it  on  any  ground.  It  is 
true  that,  having  been  -warmed,  its  relative  dryness  has 
been  increased;  but  this  condition  may  be  corrected  to 
some  extent  by  always  keeping  in  the  hot-air  chamber  of. 
the  furnace  a  vessel  of  water  for  evaporation. 

If,  however,  the  air  supplied  to  the  furnace  is  not  fresh 
and  drawn  from  the  outer  atmosphere,  but  is  simply  taken 
from  the  cellar  in  which  the  furnace  stands ;  or  if  the  fur- 
nace is  not  tight,  but  cracked  or  loose-jointed,  so  that  the 
gases  of  combustion  may  escape  and  mingle  with  the  air 
as  the  latter  flows  through  the  pipes  and  rises  into  the 
rooms  of  the  house  ;  or  if,  as  often  happens,  the  air  is  over- 
heated and  greatly  overdried,  then  furnaces  of  this  kind 
may,  and  do,  become  objectionable.  In  very  cold  and 
windy  climates,  and  for  houses  in  bleak  or  exposed  places, 
furnaces  are  less  satisfactory  than  steam  or  hot-water 
heaters.  As  they  usually  deliver  warm  air  under  very 
small  pressure,  it  is  often  impossible,  especially  in  windy 
weather,  "  to  put  the  heat  where  it  is  wanted,"  a  difficulty 
not  encountered  in  the  use  of  steam  or  hot  water.  Another 
objection  to  hot-air  furnaces  is  the  fact  that  much  dust 
finds  its  way  in  with  the  warm  air ;  but  fresh  air  without 
dust,  at  least  in  towns  and  cities,  is  rare. 

A  simple  combination  of  stove  and  furnace  is  much  used 
in  some  places,  where  a  stove  (usually  in  or  against  the 
fireplace)  on  the  first  floor  heats  not  only  the  room  in  which 
it  stands  but  also,  by  means  of  a  pipe,  or  pipes,  and  regis- 
ters, one  or  more  rooms  overhead.  Unfortunately  the  air 
thus  supplied  to  the  upper  rooms  is  not  always  pure  air 
from  out  of  doors ;  sometimes  it  is  the  already  vitiated 
air  of  the  room  below. 


WARMING  AND  LIGHTING  OF  HOUSES     437 

5.  Warming  by  Steam  and  by  Hot  Water.  —  It  is  very 
common  in  the  United  States  to  find  houses  (and  other 
buildings)  heated  by  steam  or  hot  water.    Through   the 
"  radiators  "  or  "  coils  "  placed  in  the  various  rooms  there 
is  maintained  a  circulation  of  steam  or  of  hot  water  from 
a  "heater"  below.    Here  the  room  is  warmed  partly  by 
direct  radiation  and  partly  by  convection  currents,  very 
much  as  in  the  case  of  the  stove.    The  chief  objection  to 
this  method  of  heating  is  that  the  heating  and  the  ventila- 
tion of  the  room  are  not  effected  by  the  same  process ;  the 
room  must  be  ventilated  by  opening  windows,  and  the  air 
thus  introduced  is  apt  to  be  cold  and  to  produce  undesirable 
drafts.    On  the  other  hand,  steam  and  hot  water  are  both 
superior  to  hot  air  in  convenience  and  efficiency.    They 
can  be  carried  anywhere  and  are  free  from  disturbances  by 
atmospheric  conditions,  wind-pressure,  natural  ventilation, 
etc.,  which  greatly  interfere  with  the  proper  distribution  of 
hot  air.1    Sometimes  a  combination  of  direct  and  "  indirect 
radiation  "  2  is  employed,  the  latter  being  used  for  all  ordi- 
nary heating,  and  the  former  kept  for  aid  in  extremely  cold 
or  windy  weather.    On  this  plan  fresh  air  drawn  from  out- 
side is  first  passed  over  coils  of  pipes  placed  in  a  basement 
or  cellar  and  containing  steam  or  hot  water,  and  then  car- 
ried (as  in  the  hot-air  furnace)  to  the  various  rooms  which 
it  is  desired  to  warm.    In  addition,  radiators  are  placed  in 
these  rooms,  often  near  doors  or  windows,  and  in  extreme 
cold  weather  are  charged  with  steam  or  hot  water  to  fur- 
nish supplementary  heating  by  direct  radiation. 

6.  Oil  Stoves,  Gas  Stoves,  and  Electric  Heaters.  —  These 
do  not  greatly  differ  from  other  stoves  except  in  the  sources 

1  The  hot-water  system  is  rapidly  coming  into  favor  to  replace  steam 
heat,  because  a  given  volume  of  water  will  carry  a  larger  amount  of  heat 
than  the  same  volume  of  steam  ;  consequently  the  water  can  be  sent  from 
the  heater  at  a  lower  temperature  than  the  steam,  the  supply  pipe  is  not 
so  hot,  and  the  heat  is  more  evenly  distributed  through  the  house. 

2  "Indirect  radiation"  is,  of  course,  really  convection. 


438  THE  HUMAN  MECHANISM 

of  the  heat  which  they  provide,  and  in  the  important  fact 
that  in  the  first  two  the  products  of  combustion  are  not 
usually  carried  off  by  chimneys  but,  as  in  oil  or  gas  lamps 
(which  are  also  powerful  room  warmers  and  really  only 
luminous  stoves),  escape  directly  into  the  room  and  thus 
tend  to  vitiate  its  atmosphere  without  causing  any  com- 
pensating ventilation.  In  the  case  of  gas  stoves  special 
care  must  be  taken  to  see  that  the  unburnt  gas  does  not 
escape  into  the  room  from  leaks  in  the  connections  or 
elsewhere.  Here  the  same  considerations  apply  as  in  the 
case  of  gas  used  for  lighting.  The  flexible  rubber  tubes 
often  used  for  supplying  gas  stoves  deteriorate  with  age 
and  then  frequently  permit  the  escape  of  gas  directly  into 
the  room.  Whenever  possible,  permanent  (metallic)  con- 
nection of  the  stove  with  the  iron  gas  pipe  is  advisable. 
Where  rubber  connections  are  used,  the  gas  should  always 
be  turned  off  at  the  cock  on  the  main  pipe,  never  at  that  on 
the  stove. 

Electric  stoves,  like  electric  lights,  are  heated  by  elec- 
tricity, and  even  electric  lights,  though  inferior  in  this 
respect  to  oil  or  gas  lights,  are  often  noteworthy  factors 
in  the  warming  of  houses.  Stoves  used  for  cooking  add 
materially  to  the  warmth  of  houses,  and  hence  gas  or  oil 
stoves  may  be  used  with  advantage  when,  as  in  snrnmer, 
heat  is  undesirable. 

7.  Solar  Heating.  Glass  Verandas.  —  Less  use  is  made 
of  the  direct  heat  of  the  sun  than  is  often  advisable  or  ad- 
vantageous. Rooms  flooded  with  sunshine  are  always  more 
economically  warmed  than  those  without  it,  and  a  solarium, 
or  glass-covered  room  or  veranda,  on  the  south  side  of  a 
house  is  often  useful  as  well  as  agreeable  in  winter.  If 
provision  is  made  for  heating  it  at  night,  and  in  cold  and 
cloudy  weather,  it  may  be  made  to  answer  also  as  a  plant 
conservatory,  or  greenhouse,  and  thus  become  a  source  of 
added  interest  and  pleasure. 


WARMING  AND  LIGHTING  OF  HOUSES     439 

8.  Overheating.  —  If  the  temperature  of  the  house  is  too 
high,  we  suffer  from  many  of  the  objectionable  conditions 
of  hot  weather ;   mental  work  is  more  difficult  and  we  are 
disinclined  to  muscular  exercise.    It  is  probably  unwise 
to  keep  the  temperature  of  the  house  above  68°  or  70°  F. 
A  good  rule  is  to  keep  it  between  65°  and  70°  F.  (see 
pp.  389-391).    Those  who,  by  reason  of  infirmities  of  age, 
cannot  enjoy  regular  muscular  activity  often  find  rooms 
of  this  temperature  too  cold  ;  but  the  aged  should  be  en- 
couraged and  even  urged  to  keep  up  at  all  hazards  the 
habit  of  doing  some  muscular  work  every  day.    With  care- 
ful attention  to  muscular  exercise  and  outdoor  life  they 
can  not  only  endure  but  also  enjoy  lower  room  tempera- 
tures  than  is  generally  supposed,   and  thus  permit  the 
younger  members  of  the  household  to  live   under  more 
wholesome  temperature  conditions.    Appetite  is  also  im- 
proved by  this  practice  and  old  age  made  in  general  more 
comfortable  and  more  cheerful.    Youth  should  remember, 
however,  that  the  aged,  largely  because  they  cannot  "  get 
warm  from  the  inside,"  not  only  desire  but  actually  require 
warm  clothing  and  often  very  warm  rooms. 

9.  The  Lighting  of  the  House.  —  The  fire  light  and  the 
light  of  the  pine  knot,  with  which  the  hut,  the  hovel,  or 
the  wigwam  were  lighted,  were  objectionable  chiefly  because 
of  their  inconvenience,  smoke,  and  flare  or  flicker.    The 
invention  of  the  latnp  without  chimney,  and  of  the  candle, 
marked  a  step  forward,  though  their  light  was  weak  and 
flickering.    A  much  greater  advance  was  the  invention  of 
the  lamp  chimney,  as  it  provided  what  nothing  else  had 
done,  steadiness  of  flame,  and  avoided  flare  and  flicker. 
Once  the  latter  was  overcome,  it  became  easy  to  improve 
the  fuel,  until  now  the  oil  lamp  with  chimney  not  only 
illuminates   and  decorates   the  home   of  wealth  but  also 
brightens  and  cheers  the  hut  of  the  fisherman  and  the  cabin 
of  the  sailor;  it  aids  and  comforts  the  seamstress  in  her 


440  THE  HUMAN  MECHANISM 

toil,  in  the  humblest  lodging ;  it  warns  the  mariner  by 
night  from  dangerous  coasts  by  lighthouses,  and  throws 
about  the  student  a  warm  and  cheerful  radiance  as  he 
"burns  the  midnight  oil."  Candles  are  still  much  used 
both  in  churches  and  in  houses,  but  chiefly  because  of  sen- 
timent or  for  decoration.  They  still  furnish  the  softest 
and  most  beautiful  light,  especially  for  quiet  places;  but 
they  are  unfit  for  reading  purposes  because  of  their  flicker- 
ing and  their  feebleness. 

The  introduction  of  gas  lighting  was  a  great  advance 
over  lighting  by  lamps,  owing  to  its  convenience  and  clean- 
liness and  the  intensity  of  the  light  afforded.  But  gas 
lights,  unless  provided  with  chimneys,  are  generally  un- 
steady and  therefore  objectionable  for  use  in  reading.  Gas 
lights  (and  oil  lamps)  also  produce  much  heat,  and  by 
this  as  well  as  by  their  products  of  combustion  may  greatly 
vitiate  the  air  of  rooms  in  which  they  are  used. 

While  some  kinds  of  illuminating  gas  are  more  poison- 
ous than  others,  all  manufactured  —  as  distinguished  from 
"  natural  "  —  gas  contains  a  considerable  percentage  of  poi- 
sonous constituents.  When  the  gas  is  burned,  these  are 
oxidized  and  form  harmless  substances,  and  hence  there 
is  little  or  no  danger  from  the  products  of  its  combustion. 
But  the  greatest  care  should  be  taken  to  avoid  the  entrance 
in  any  way  of  unburned  gas  into  the  air  of  a  room.  This 
may  happen  by  the  gas  escaping  through  leaky  fixtures,  or 
after  "  blowing  it  out "  instead  of  turning  it  off.  It  may 
also  occur  when  the  light  has  been  turned  down  very  low  in 
the  sleeping  room  and  is  afterwards  blown  out  by  a  draft,  or 
goes  out  because  of  lessened  pressure  in  the  main,  and  the 
unburned  gas  escapes  freely  when  the  pressure  is  restored. 
Still  another  source  of  danger  exists  when  the  gas  cock 
used  to  turn  off  the  gas  works  too  easily  in  its  socket,  and 
so  is  capable  of  being  turned  on  by  slight  jars,  touches, 
etc.  The  student  is  referred  to  Chapter  XXXII  for  a  full 


WAKMING  AND  LIGHTING  OF  HOUSES     441 

description  of  the  dangers  of  inhaling  unburned  illuminating 
gas.  Illuminating  gas  is  also  explosive  when  mixed  with 
air  in  certain  proportions. 

Electric  lighting  is  in  many  respects  an  ideal  method, 
giving  a  convenient,  steady,  and  powerful  light ;  but,  as  is 
stated  in  the  next  paragraph,  care  must  be  exercised  that 
such  light  is  not  too  bright. 

10.  The  Best  Light.  —  Probably  there  is  no  one  kind  of 
light  which  is  best  for  all  purposes.  For  general  illumi- 
nation of  public  squares  and  public  buildings  the  electric 
light  seems  to  be  generally  preferred.  The  same  thing 
is  probably  true  of  private  houses.  For  reading  and  for 
microscopic  work,  on  the  other  hand,  the  electric  light 
may  easily  be  too  bright ;  but  this  objection  can  be  over- 
come by  using  lamps  of  proper  candle  power,  by  having 
the  lamp  at  a  suitable  distance,  or  by  using  bulbs  with 
ground  glass.  The  same  thing  may  be  true  of  the  light 
yielded  by  any  incandescent  solid,  such  as  the  "  lime " 
(oxy hydrogen)  light  and  the  various  "  mantles "  made 
from  incombustible  earths,  such  as  that  in  the  Welsbach 
light.  In  general,  for  reading  a  "  soft "  light  is  best,  and 
it  is  desirable  to  have  the  larger  part  of  the  light  come  to 
the  book  by  reflection  from  the  walls  of  the  room  rather 
than  solely  and  directly  from  any  source  of  light  near  by. 
For  this  reason,  dark-colored  walls  are  objectionable  for 
rooms  in  which  a  number  of  people  do  much  reading, 
sewing,  or  other  near  work. 


CHAPTER  XXVIII 
THE  AIR  SUPPLY  OF  THE  HOUSE.    VENTILATION 

Besides  the  relatively  permanent  furnishings  and  fixtures 
of  the  house  there  are  other  necessaries  of  civilized  domestic 
life,  such  as  air,  water,  oil,  gas,  coal,  and  provisions,  which 
come  into  the  house  only  to  be  consumed,  their  waste  ma- 
terials being  cast  out  again.  These  are  commonly  called 
the  domestic  supplies,  —  air  supply,  water  supply,  gas  sup- 
ply, etc.,  —  and  they  are  usually  derived  from  much  larger 
public  supplies  which  are  used  in  common  by  many  fami- 
lies. All  such  public  supplies,  although  convenient,  may, 
under  certain  circumstances,  become  dangerous  to  human 
life  and  health. 

1.  The  Air  Supply  of  the  house  is  probably  more  neg- 
lected than  any  other.  Water,  gas,  coal,  and  provisions  are 
costly  and  often  difficult  to  get,  but  air  is  always  abundant 
and  cheap.  The  familiar  saying,  "  as  free  as  air,"  applies 
best,  however,  to  outdoor  air;  for,  as  we  shall  see,  good 
air  in  houses  is  not  always  very  abundant,  nor  always  cheap 
and  easy  to  provide. 

Inasmuch  as  the  adult  human  body  requires  for  its 
regular  uses  about  five  hundred  cubic  inches  of  air  per 
minute,  the  air  in  the  immediate  vicinity  of  the  nose  is 
quickly  used  up ;  and  as  an  equal  amount  of  exhausted 
or  vitiated  air  is  discharged  per  minute  at  the  same  place, 
the  need  is  obvious  of  a  constant  streaming  of  air  about  the 
body  which  shall  remove  vitiated  air  and  supply  fresh  air. 
This  circulation  or  flow  of  air  is  just  as  necessary  as  is  the 
circulation  of  the  blood  ;  but,  as  the  movement  always 

442 


VENTILATION  443 

goes  on  unseen,  through  the  diffusion  of  gases  and  by 
other  natural  and  invisible  agencies,  it  is  harder  to  realize 
the  need.  Out  of  doors  the  air  supply  is  ordinarily  suffi- 
cient and  of  good  quality,  especially  while  the  body  is 
in  motion,  in  walking,  driving,  riding,  wheeling,  skating, 
sailing,  rowing,  or  in  doing  much  of  the  manual  work  on 
farms,  forests,  ships,  in  fishing,  etc. 

2.  Stagnant  Air.  —  Indoors, conditions  are  very  different. 
Life  is  more  sedentary,  the  body  is  more  quiet,  and  nat- 
ural wind  currents  or  drafts  are  intentionally  prevented ; 
the  air  of  houses  tends  to  become  stationary  and  even 
stagnant,  and  with  it  the  air  supply  about  the  bodies  of 
the  house  dwellers.    Since  it  is  this  stagnant  air  which  is 
steadily  exhausted  or  vitiated  by  the  air  discharged  from 
the  nose  and  mouth,  a  blanket  of  increasingly  stagnant 
and  impure  air  tends  to  accumulate  about  the  body  of  a 
sitting  or  sleeping  person.    To  prevent  this   stagnation, 
and  the  consequent  impurity  of  the  air  supply,  movement 
of  the  body  or,  better,   movement  of  the  air  is  a  prime 
necessity. 

3.  Ventilation  (Latin,  ventus,  wind)  is  the  name  usually 
given  to  any  circulation  or  movement  of  the  air  of  rooms 
or  buildings  by  which  fresh,  pure  air,  preferably  from  out- 
doors, is  introduced  and  vitiated  air  is  removed,  the  move- 
ment of  the  air  being  rapid  enough  to  meet  all  the  needs 
of  the  body,  but  not  so  rapid  as  to  cause  dangerous  cur- 
rents  or  drafts.    This    movement  or  circulation  may  be 
either  intermittent  and  occasional,  as  when  a  window  is 
opened  for  a  little  while  and  then  shut,  or  more  or  less 
regular  and  constant,  as  in  all  efficient  "  systems  "  of  ven- 
tilation or  even  in  such  primitive  methods  as  that  of  the 
chimney  above  an  open  fire. 

4.  Natural  Ventilation.  —  The  walls  of  most  houses  are 
more  or  less  porous  and  permeable  for  gases.    Cracks  and 
crevices   around   doors  and  windows  also  allow  gases  to 


444  THE  HUMAN  MECHANISM 

leave  and  enter.  In  an  experiment  made  by  one  of  the 
authors,  four  ordinary  gas  jets  in  a  small  room  were  left 
open  (but  not  lighted)  all  night,  and  after  the  gas  had 
poured  in  for  eight  hours  it  was  found  that  the  room  con- 
tained only  three  per  cent  of  gas,  the  remainder  having 
escaped  by  natural  ventilation.  It  is  largely  because  of  the 
cracks,  crevices,  and  pores  in  the  walls  that  human  beings 
get  on  as  well  as  they  do  in  rooms  and  houses  seemingly 
wholly  unventilated.  Wood,  brick,  stone,  and  plaster  are 
more  porous  than  glass,  iron,  or  glazed  brick,  and  dry  walls 
more  porous  than  those  wet  or  damp.  Painted  and  papered 
walls  are  less  porous  than  those  left  bare,  and  accordingly 
the  walls  of  summer  houses  are  often  loosely  made,  prefer- 
ably of  "  natural  "  woods. 

5.  What  do  we  mean  by  Air  Good  or  Bad,  Pure  or 
Impure  ?  —  Air  is  not  a  chemical  compound  of  fixed  com- 
position, but  a  mixture  of  gases  containing,  even  when 
pure,  varying  amounts  of  nitrogen,  oxygen,  carbonic  acid, 
ammonia,  and  water,  —  this  last  in  the  form  of  invisible 
(aqueous)  vapor.  Moreover,  the  density  of  the  air  varies 
not  only  at  different  places,  but  also  at  the  same  place  at 
different  times.  Impure  air  contains  all  these  gases,  and 
may  contain  in  addition  any  other  gas  capable  of  mixing 
with  them,  such  as  hydrogen  sulphide,  carbonic  oxide, 
marsh  gas,  etc.  The  terms  "good"  and  "bad"  air, 
"  moist,"  "  fine,"  "  dry,"  "  bracing,"  "  muggy,"  "  humid," 
"heavy,"  "foul,"  "fetid,"  "stagnant,"  "dead,"  "thick," 
or  "lifeless"  air,  and  all  similar  terms,  are  popular  de- 
scriptions of  atmospheric  conditions  real  or  imaginary, 
testifying  to  the  wonderful  variety  of  this  part  of  the 
environment  of  mankind. 

We  may  define  "  pure  "  air  as  any  portion  of  the  atmos- 
phere free  from  noxious  gases  or  vapors  and  from  infec- 
tious microorganisms.  Such  air  may,  however,  be  unfit 
for  breathing,  as  is  the  case  with  those  higher  portions 


VENTILATION  445 

of  our  atmospheric  ocean  into  which  aeronauts  have  some- 
times ventured.  At  the  height  of  three  miles  above  the 
sea  the  air  no  doubt  is  very  u  pure,"  but  yet  too  thin  to 
support  human  life  readily. 

Air  may  be  considered  as  polluted  or  "  impure  "  when 
it  contains  noxious  gases,  or  floating  particles  in  large 
numbers  (as  in  smoke),  or  disease-producing  germs  or 
microorganisms  (perhaps  derived  from  dust). 

6.  The  Sources  of  Discomfort  and  Danger  in  Air.  —  We 
must  be  careful  to  discriminate  between  discomfort  and 
danger  in  atmospheric  conditions.  Positive  danger  comes 
chiefly  from  deficiency  of  oxygen,  excess  of  carbonic  acid, 
admixture  of  poisonous  gases,  or  from  infectious  micro- 
organisms. Aeronauts,  explorers  on  high  mountains,  and 
persons  living  at  great  altitudes  are  apt  to  suffer  from 
oxygen  deficiency.  Miners,  charcoal  burners,  and  well 
cleaners  sometimes  suffer  from  carbonic  acid  excess. 
Laborers  in  gas  works  and  consumers  of  illuminating  gas 
may  be  poisoned  by  carbon  monoxide ;  and  workers  in 
sewers,  by  various  gases,  especially  by  illuminating  gas 
which  may  have  leaked  in.  Air  may  also  contain,  and 
thus  conv.ey,  germs  of  infectious  diseases  such  as  smallpox, 
scarlet  fever,  measles,  etc. 

On  the  other  hand,  air  that  is  not  dangerous,  and  even 
perfectly  "  pure  "  air,  may  be  a  source  of  great  discomfort, 
simply  because  of  its  temperature  or  moisture,  or  its  tem- 
perature and  moisture  taken  together.  The  air  in  the 
"dog  days"  of  August  is  no  less  pure  than«that  of  June 
or  October;  yet  it  is  often  oppressive  because  it  is  both 
too  warm  and  too  moist.  It  has  been  shown  in  Chapter  XII 
how  greatly  the  regulation  of  the  temperature  of  the  body 
depends  upon  the  capacity  of  the  atmosphere  to  take  up 
moisture,  and  it  is  plain  that  any  atmosphere  saturated  or 
nearly  saturated  with  aqueous  vapor  must  seriously  inter- 
fere with  the  cooling  of  the  body.  A  careful  review  of  that 


446  THE  HUMAN  MECHANISM 

chapter  will  greatly  help  the  student  to  an  understanding 
of  the  sources  of  discomfort  in  the  atmosphere  of  houses 
or  rooms. 

A  shut  and  uninhabited  room  often  becomes  "  musty" 
or  "  damp,"  because  of  a  want  of  circulation  to  remove  air 
containing  traces  of  odoriferous  gases  and  excess  of  mois- 
ture ;  the  former  perhaps  derived  from  carpets  or  furniture, 
the  latter  from  the  basement  or  cellar. 

The  air  of  an  inhabited  room  may  prove  a  source  of  dis- 
comfort to  its  inmates,  and  therefore  deserve  to  be  called 
lad  for  any  or  all  of  the  following  reasons:  (1)  the  air 
may  be  overheated  or  underheated ;  (2)  it  may  contain  an 
excess  of  moisture  due  either  to  its  dampness  of  location 
or  to  the  breath  of  its  inmates;  (3)  it  may  be  both  over- 
heated and  overmoist ;  (4)  it  may  contain  odoriferous  gases 
which  cause  displeasure  or  discomfort. 

What  such  rooms  do  not  often  suffer  from  is  oxygen  defi- 
ciency or  carbonic  acid  excess;  for  experiments  have  proved 
that  unless  the  oxygen  falls  below  twelve  per  cent,  or  the 
carbonic  acid  rises  above  three  per  cent  (conditions  which 
are  very  rarely  met  with  in  ordinary  human  habitations), 
no  marked  discomfort  ensues. 

7.  Ventilation  replaces  Bad  Air  with  Good  Air  and  causes 

Aerial  Movement  or  Circulation It  is  now  easy  to  see 

precisely  how  ventilation  aids  us  in  securing  comfortable 
and  agreeable  atmospheric  conditions.  It  removes  bad  air 
and  supplies  good  (that  is  fresh)  air  and,  by  causing  move- 
ment, favor*  evaporation  from  the  skin  and  consequent 
cooling  on  muggy  days.  It  is  also  easy  to  see  why  ventila- 
tion is  at  times  ineffective.  No  system  of  ventilation  can 
wholly  overcome  the  "  mugginess  "  of  a  "close"  room  in 
August,  because  the  pure  outer  air  is  itself  unpleasant 
and  uncomfortable ;  but  active  ventilation,  by  producing 
a  breeze,  can  do  more  than  anything  else  to  make  the  con- 
ditions tolerable  (see  Part  I,  p.  203). 


VENTILATION  447 

8.  Fans  and  Fanning.  —  It  is  an  old  point  of  dispute 
whether  or  not  a  person  who  "  fans  "  himself  grows  cooler 
or  warmer.    However  this  may  be,  there  can  be  no  question 
that  persons  who  use  fans  feel  cooler,  and  there  is  no  doubt 
that  any  one  fanned  by  another  or  by  a  breeze  not  only 
feels,  but  actually  is,  cooled  thereby.    The  great  and  grow- 
ing use  of  electric  fans  in  hotels,  houses,  etc.,  testifies  to 
the  same  fact. 

9.  A  Room  may  be  well  Ventilated  but  Oppressive  from 
Overheating. —  This  fact,  though  perfectly  obvious,  and  fa- 
miliar to  all  who  have  been  in  well-ventilated  boiler  rooms, 
or  who  have  lived  in  the  tropics,  is  too  little  attended  to. 
Many  public  halls,  Pullman  and  other  railway  cars,  steam- 
boats, and  private  houses,  especially  in  the  northern  United 
States,  are  rendered  almost  intolerable  and  very  unhygienic 
by  simple  overheating.    Elderly  people  and  infants  require 
higher  room  temperatures  than  do  active  persons  in  youth 
and  middle  life,  but  in  general  any  temperature  above  70°F. 
must  be  regarded  as  excessive,  and  65°F.  to  68°F.  is  a  better 
temperature  (pp.  201,  390).    Housekeepers,  at  least  in  the 
northern  United  States,  would  do  well  to  try  to  keep  the 
mercury  in  their  houses  between  these  lower  limits.    When 
the  outdoor  air  is  moist,  as  in  rainy  weather,  a  somewhat 
higher  temperature  is  often  required  than  when  it  is  dry. 

10.  A  Room  may  be  Comfortable  in  Temperature  but 
Defective  in  Ventilation.  —  This  fact  is  less  obvious  than 
that  just  considered,  but  nevertheless   true.    It  may  be 
because  of  excessive  moisture,  or  because  of  odors,  or  for 
other  reasons ;  but  those  entering  such  rooms  from  out  of 
doors  are  often  able  to  remark  and  deplore  the  fact.    It  is 
perhaps  oftenest  exemplified  in  warm  countries  or  in  warm 
seasons  when  people  close  and  darken  rooms  to  "  keep  out 
the  heat."    Such  closing  and  darkening  interferes  with  a 
good  circulation  of  air  and  thus   hinders  ventilation.    It 
also  favors  dampness  by  excluding  sunshine. 


448  THE  HUMAN  MECHANISM 

11.  Some  Practical  Hints  about  the  Ventilation  of  Rooms. 
—  We  have  already  referred  to  the  great  value  of  chimneys 
and  open  fireplaces  as  ventilators,  and  to  the  "natural" 
ventilation  by  porous  walls,  and  by  cracks  above  and  below 
doors  and  windows.  The  simplest  and  most  usual  artificial 
method  of  ventilation  is  the  opening  more  or  less  widely 
of  windows  or  doors,  and  this  is  a  means  which  should 
never  be  disregarded.  The  great  drawback  associated  with 
it  is  the  fact  that  uncomfortable  and  frequently  unwhole- 
some drafts  are  likely  to  ensue.  It  should  be  remem- 
bered, however,  that  the  existence  of  a  decided  draft  is 
usually  an  indication  that  the  amount  of  ventilation  is 
greater  than  is  necessary.  When  the  wind  is  blowing 
against  a  window  it  is  enough  to  open  this  an  inch  or 
less ;  if  the  wind  is  blowing  very  hard,  the  natural  ventila- 
tion may  be  sufficient.  Moreover,  the  amount  of  natural 
ventilation  secured  depends  quite  as  much  on  the  ease  of 
egress  as  of  ingress  of  air.  It  often  happens  that  if  the 
window  be  closed  or  very  slightly  opened  on  the  wind- 
ward side  of  the  house,  enough  natural  ventilation  will 
be  secured  by  opening  other  windows  on  the  side  away 
from  the  wind. 

It  is  often  possible,  especially  in  warm  or  temperate 
weather,  to  secure  satisfactory  ventilation  by  opening  win- 
dows both  at  the  top  and  the  bottom,  the  warm  air  passing 
out  above,  while  cooler,  fresh  air  comes  in  below.  This, 
however,  is  not  advisable  when  the  temperature  .of  the 
incoming  air  is  too  low,  since  the  air  then  sinks  at  once 
to  the  floor  and  chills  the  feet.  Another  good  plan  is  to 
raise  the  lower  sash  three  or  four  inches  and  place  under 
it  a  board  made  to  fit  the  space.  Air  now  enters  between 
the  sashes  and,  the  air  being  directed  upward,  the  occu- 
pants of  the  room  are  protected  from  drafts.  Where  elec- 
tricity is  available,  an  electric  fan  placed  in  one  of  the 
upper  sashes  is  frequently  very  effective  in  hastening  the 


VENTILATION  449 

removal  of  vitiated  air.  Fans  for  this  purpose  are  now  read- 
ily obtainable,  and  have  often  proved  to  be  serviceable. 

In  winter,  fresh  air  and  good  ventilation  cost  something, 
as  the  air  must  be  heated ;  but  it  is  poor  economy  to  use 
stagnant  air  for  the  sake  of  saving  fuel.  The  keen  edge 
of  capacity  for  good  work  is  dulled  by  bad  air,  the  vital 
resistance  is  lowered,  and  the  susceptibility  to  disease  in- 
creased. On  the  other  hand,  there  is  such  a  thing  as  too 
much  ventilation ;  for  if  it  causes  dangerous  drafts  or 
leads  to  actual  chilling  of  the  body,  it  may  do  almost  as 
much  harm  as  too  little  ventilation.  Here  again  each  in- 
dividual must  study  and  determine  his  own  needs. 

The  hot-air  furnace  is  capable  of  supplying  fresh  air  in 
abundance  and,  if  the  air  be  not  overheated  or  overdried, 
gives  an  admirable  method  of  heating  and  ventiration, 
combined  in  a  single  device,  —  the  jacketed  stove,  —  pro- 
vided always  that  the  air  supplied  to  its  heating  chamber 
is  unobjectionable. 

12.  Mechanical  Systems  of  Ventilation.  -  -  Buildings 
larger  than  dwelling  houses,  such  as  large  schoolhouses  or 
public  halls,  are,  or  should  be,  ventilated  by  some  mechan- 
ical system.  These  are  of  two  principal  types,  known  as 
the  "  vacuum  "  and  the  "  plenum  "  systems.  In  the  former 
an  attempt  is  made  to  effect  good  ventilation  by  sucking 
out  the  air  from  the  building  by  an  exhaust  fan  or  blower 
attached  to  one  main  pipe  or  duct,  to  which  are  led  tribu- 
tary ducts  connected  with  the  various  rooms.  To  supply 
the  air  thus  removed,  fresh  air  is  supposed  to  make  its 
way  in,  either  by  "  natural "  ventilation  (p.  443)  or  through 
inlet  ducts  specially  provided,  in  either  case  being  pressed 
in  by  the  outer  atmospheric  pressure.  In  the  plenum 
system  this  arrangement  is  reversed,  and  air,  previously 
warmed  if  need  be,  is  driven  by  a  fan  into  a  main  duct 
or  space,  from  which  smaller  ducts  carry  it  to  the  various 
rooms,  circulation  being  favored  by  outlet  ducts  through 


450  THE  HUMAN  MECHANISM 

which  the  air  flows  away.  Sometimes  an  effective  combi- 
nation of  the  two  systems  is  used,  in  which  case  the  air  is 
not  only  pumped  in  but  also  at  the  same  time  sucked  out. 

In  favor  of  the  plenum  system  it  may  be  said  that, 
instead  of  currents  of  (often)  cold  air  pressing  in  by  the 
paths  of  natural  ventilation,  about  doors,  windows,  etc., 
the  direction  of  the  aerial  current  at  these  places  is  re- 
versed, owing  to  the  pressure  to  which  the  air  is  subjected, 
so  that  it  is  more  often  possible  to  sit  very  near  such  win- 
dows and  doors. 

The  combination  of  the  two  systems  offers  many  prac- 
tical advantages,  but  is  obviously  relatively  costly.  It  is 
sometimes  forgotten  that  air,  quite  as  truly  as  water,  pos- 
sesses inertia  and  moves  along  paths  of  least  resistance ; 
but  experience  has  shown  that  in  order  to  govern  the 
direction  of  flow  of  the  aerial  stream  it  is  often  necessary, 
as  well  as  advisable,  to  control  the  outgo  as  well  as  the 
income  of  air. 


CHAPTER  XXIX 

THE  WATER  SUPPLY,  PLUMBING,  AND  DRAINAGE  OF  THE 
HOUSE.    GARBAGE  AND  RUBBISH 

1.  Water  Supply.  —  The  water  supply  of  the  house 
should  be  first  pure,  and  second,  abundant.  No  exact 
figures  can  be  given  as  to  the  amount  required,  but  for 
kitchen  and  laundry  use,  bathing,  and  good  drainage,  it 
is  safe  to  say  that  thirty  gallons  per  day  per  capita  are 
ample.  An  ordinary  barrel  holds  this  amount.  Most  fam- 
ilies get  on  with  very  much  less ;  but  for  the  greatest 
convenience  and  cleanliness  some  such  quantity,  if  not  ab- 
solutely needed,  can  be  used  to  advantage,  and  no  domes- 
tic supply  is  a  greater  luxury  than  abundant  water. 

The  purity  of  the  domestic  water  supply  should  be 
above  suspicion.  In  a  following  chapter  we  shall  empha- 
size (Chapter  XXXII)  the  requirements  as  to  purity  of  a 
proper  public  water  supply  from  which  the  domestic  sup- 
ply may  be  drawn  ;  but  we  may  here  consider  briefly  those 
private  supplies,  such  as  wells,  springs,  and  brooks,  from 
which  many  houses,  especially  in  the  country,  must  obtain 
their  water.  It  is  worth  remembering  that  all  water  in 
or  upon  the  earth  was  originally  rain  water  (i.e.  distilled 
water  from  the  atmosphere).  This,  when  it  flows  over  the 
surface  of  the  earth,  or  percolates  through  the  ground,  is 
known  later  as  surface  water,  or  ground  water.  Streams, 
such  as  brooks,  creeks,  and  rivers,  are  composed  largely, 
but  by  no  means  wholly,  of  surface  water;  deep  wells, 
dug  or  driven,  and  many  springs,  contain  a  mixture  of  sur- 
face and  ground  waters.  Surface  waters  are  particularly 

451 


452  THE  HUMAN  MECHANISM 

exposed  to  pollution  by  dirt  and  filth  from  roads,  manured 
fields,  arid  the  surface  of  the  ground  generally.  Ground 
waters,  on  the  other  hand,  although  subject  to  pollution 
by  percolating  through  buried  filth,  and  by  surface  waters 
mingling  with  them  through  cracks  or  fissures  in  the  earth, 
are,  in  general,  subject  to  great  purification  ~by  filtration 
during  their  peroolation  through  earth,  which  often  acts  as 
a  porous  filter  (Fig.  116). 

2.  Domestic  Wells.  —  The  well  of  water  has  an  ancient 
reputation  and  has  long  been  celebrated  in  song  and  story. 
As  a  supply  more  or  less  public  it  has  often  served  as  a 
meeting  place  and  a  social  center,  and  has  frequently  been 
ornamented  with  decorated  curbs  or  covers  testifying  to 
popular  esteem.  Until  1854  the  common  well  was,  with 
rare  exceptions,  regarded  as  a  perfectly  safe  and  satisfac- 
tory method  of  securing  water  for  public  as  well  as  pri- 
vate water  supplies ;  but  in  that  year  all  wells  began  to 
be  regarded  with  suspicion,  because  of  a  disastrous  out- 
break of  Asiatic  cholera  in  London,  which  was  conclusively 
traced  to  a  polluted  public  well  on  Broad  Street  in  that 
city.  It  was  found  on  investigation  that  a  privy  vault, 
probably  infected  by  the  discharges  of  a  cholera  patient, 
had  leaked  directly  into  the  well;  and  immediately  all 
wells,  especially  those  near  any  source  of  pollution,  fell 
under  suspicion. 

The  truth,  in  brief,  appears  to  be  that  many  wells 
are  absolutely  innocent  of  all  contamination  and  yield 
excellent  water.  Some  wells  undoubtedly  contain  water 
originally  impure,  but  coming  from  a  long  distance  through 
the  soil  before  reaching  the  well  and  thoroughly  purified 
by  filtration.  Others,  however,  are  in  more  direct  con- 
nection with  cesspools,  privy  vaults,  barnyards,  stables, 
or  similar  objectionable  and  perhaps  dangerous  sources, 
and  are  utterly  unfit  to  serve  as  water  supplies  for  domes- 
tic uses.  Still  others,  though  receiving  good  water  from 


WATER  SUPPLY  AND  DRAINAGE 


453 


the  earth  about  them,  yield  bad  water  because  objection- 
able matters  find  their  way  in  at  the  top.  Poultry  should 
be  prevented  from  walking  over  loose  planking  which  only 
partially  covers  the  well,  and  farmers  whose  boots  have 


FIG.  116.  A  domestic  well  badly  situated  in  a  farmyard 

Observe  that  the  water  which  it  yields  is  partly  drainage  from  the  barnyard; 
privy,  pigpen,  etc.    It  is  also  exposed  to  pollution  at  the  top 

become  fouled  by  walking  in  barnyards,  or  on  fields  heav- 
ily manured  with  stable  manure,  should  be  careful  to  avoid 
doing  likewise.  The  danger  of  even  worse  contamination 
of  wells  from  manure  or  other  surface  dirt  washed  in  at 
the  top  during  heavy  rains  is  also  very  great  (Figs.  116 
and  117). 

3.  Springs  are  usually  sources  of  pure  water,  but  a  spring 
in  a  barnyard  or  a  cemetery  would  be  plainly  objection- 
able, and  care  should  always  be  taken  to  ask  whence  comes 
the  water  which  the  spring  yields.  Springs  often  occur  on 
hillsides;  in  such  cases  they  should  be  protected  from  the 
possibility  of  surface  pollution,  while  sources  of  pollution 
of  any  sort  higher  up  the  hill  should  not  be  tolerated, 
since  from  these  the  spring  may  become  contaminated. 


454  THE  HUMAN  MECHANISM 

4.  Cisterns  of  Rain  Water  are  often  used  for  domestic 
supply  in   country  houses   and  in  some  places,  such  as 
New  Orleans  and  the  Bermuda  Islands,  where  wells  are 
not  available.    There  is  no  objection  to  this  practice,  which 
should  secure  a  pure  and  very  soft  water,  provided  the 
roofs,  cisterns,  reservoirs,  and  other  receptacles  employed 
in  collection  or  storage  of  the  water  are  clean  and  suitable. 
Painted  roofs,  and  pipes,  roofs,  or  reservoirs  containing 
any  exposed  metallic  copper  or  lead  should  be  avoided, 
since  rain  water  may  attack  these  metals,  forming  with 
them  soluble  and  poisonous  salts. 

After  a  long  dry  period  in  summer,  roofs  are  often  dusty 
and  dirty,  and  the  first  washings  of  dirty  roofs  should  be 
allowed  to  go  to  waste.  Rain  water  collected  in  winter 
or  spring,  and  subjected  to  long  storage,  is  probably  the 
purest  and  most  desirable  cistern  water. 

5.  Streams,  such  as  brooks  and  creeks,  are  sometimes 
used  as  sources  of  private  domestic  water  supply,  and  if 
the  places  which  they  drain  are  wooded  and  entirely  unin- 
habited, i.e.  not  manured  in  tillage  or  pasturage,  the  sur- 
face water  which  they  yield  may  answer  fairly  well  for 
house  use.    But,  even  at  its  best,  such  water  is  exposed 
to  pollution  by  wild  animals  and  by  passing  tramps,  fisher- 
men, or  gunners,  and  a  carefully  protected  well  or  spring 
is,  as  a  rule,  a  safer  supply.    Well  water  is  often  more 
palatable,  especially  in    summer,  but  is  sometimes  very 
hard.    For  washing,   surface   or  rain   water   is  generally 
softer  and  of  course  unobjectionable. 

6.  Hard  Waters  and  Soft.  —  Rain  water  contains  few  or 
no  salts  in  solution,  and  is  therefore  called  "soft"  water. 
Many  surface  waters  and  some  well  and  spring  waters 
are  also  soft.    All  such  waters  readily  dissolve  soap,  and 
because  soapy  waters  are  sticky  and  easily  form  air  bubbles, 
a  "lather"  or  "soapsuds"  is  easily  made  in  soft  waters 
with  a  very  little  soap. 


WATEK  SUPPLY  AND  DRAINAGE  455 

Other  waters,  and  especially  ground  waters,  contain 
salts  in  solution,  some  of  which,  notably  those  of  calcium 
and  magnesium,  form  compounds  and  even  precipitates 
with  soap,  thus  removing  it  from  the  water  in  which  it 
is  placed.  Such  waters,  therefore,  require  more  soap  to 
make  them  soapy,  lathery,  or  sudsy,  and  are  known  as 
"  hard  "  waters,  because  they  feel  less  bland,  or  soft,  to  the 
skin. 

In  some  parts  of  the  United  States  the  well  waters  (and 
sometimes  even  the  surface  waters)  are  so  hard  as  to  be 
almost  or  quite  useless  for  washing,  and  even  for  drinking. 
It  has  never  been  shown  that  moderately  hard  waters  are 
necessarily  any  more  harmful  for  drinking  than  soft  waters. 
Persons  used  to  either  kind  are  apt  to  suffer  temporary  dis- 
turbances, such  as  diarrhea,  when  they  change  suddenly 
from  one  to  the  other;  but  otherwise  no  great  or  perma- 
nent harm  ordinarily  happens.  If,  however,  a  drinking 
water  is  very  hard  and  heavily  charged  with  mineral  salts 
so  that  it  becomes  essentially  a  mineral  water,  it  may  be 
unfit  for  regular  use. 

7.  House  Filters  for  water  are  not  needed  if  the  water 
supply  is  pure  and  colorless,  but  in  many  places  this  is 
not  the  case.    If  the  water  supply  is  impure,  it  should 
either  be  carefully  filtered  by  a  germ-proof  filter  (several 
kinds  of  which  are  on  the  market,  but  all  of  which  are 
costly),  or  else  boiled  for  a  few  minutes  and  cooled  before 
it  is  used  for  drinking.    If  the  water  is  pure  but  colored 
or  turbid,  it  may  be  made  bright  and  attractive  by  filtering 
through  a  charcoal  filter;   but  this   also,  if  durable  and 
effective,  is  sometimes  costly. 

8.  The  Ice  Supply  of  the  house  is  one  of  the  greatest 
of  modern  conveniences.    Ice  in  summer  was  formerly  a 
luxury,  but  in  northern  latitudes  ice  is  now  generally  har- 
vested in  winter  and  stored  for  the  following  summer.    In 
warmer  climates  the  so-called  artificial  or  manufactured  ice 


456  THE  HUMAN  MECHANISM 

brings  the  same  luxury  within  reach  of  persons  of  mod- 
erate means.  Provided  the  water  from  which  it  is  made 
is  pure,  manufactured  ice  is  as  wholesome  as  the  best 
natural  ice.  The  economical  value  of  ice  in  preserving 
foods  is  very  great,  as  is  also  its  sanitary  importance  in 
hindering  harmful  decomposition  and  decay,  for  example, 
in  milk. 

Ice  water,  so  generally  used  as  a  beverage  in  America, 
is  probably  harmless  enough  when  not  drunk  in  too  large 
quantities  or  too  rapidly;  although,  as  a  matter  of  fact, 
thirst  is  normally  slaked  by  cool  water  more  effectively 
than  by  very  cold  water.  The  ice  added  to  drinking  water 
should  be  pure ;  that  is,  ice  obtained  from  ponds,  streams, 
or  other  waters  unfit  to  serve  as  sources  of  domestic  water 
supply,  should  never  be  used  in  water  intended  for  drink- 
ing purposes,  and  all  ice  should  be  carefully  washed  before 
being  so  used. 

9.  The  Plumbing  of  the  House.  —  Almost  all  houses 
have  a  sink  of  some  sort;  from  this  there  runs  a  drain- 
pipe, which  should  be  tight,  and  large  enough  to  carry  off 
readily  the  drainage  from  the  sink.  Many  houses  have  in 
addition  more  or  less  complex  systems  of  water  supply 
and  drainage,  requiring  piping  and  plumbing. 

The  plumbing  for  water  calls  for  brief  comment  only. 
Lead  service  pipes  should,  as  a  rule,  be  avoided,  for  expe- 
rience has  shown  that  if  the  water  passing  through  lead 
pipes  happens  to  contain  an  excess  of  free  carbonic  acid 
(CO2),  this  may  attack  the  lead  and  form  with  it  a  very 
soluble  bicarbonate  which  is  a  dangerous  poison.  In  Massa- 
chusetts there  have  been  several  epidemics  of  lead  poison- 
ing due  to  this  cause. 

Service  pipes,  i.e.  pipes  leading  from  street  mains  into 
houses,  may  often  be  made  of  lead  without  any  bad  con- 
sequences, and  are  usually  preferred  by  plumbers  because 
they  are  more  easily  worked;  but  it  is  much  safer  to  use 


WATEK  SUPPLY  AND  DKA1NAGE  457 

iron  for  all  water  pipes,  although  iron  pipes  are  less  con- 
venient to  install  and  sometimes  get  badly  clogged  with 
rust. 

The  plumbing  for  drainage  should  aim  to  provide  against 
escape  or  leakage  of  both  liquids  and  gases.  As  drain- 
pipes are  not  usually  filled  with  liquids  or  gases  under 
pressure,  leaky  joints  arid  even  small  holes  may,  and  often 
do,  occur  without  detection.  If  under  such  circumstances 
any  stoppage  happens,  pressure  may  arise  and  the  liquid 
or  gaseous  contents  escape.  It  was  formerly  believed  that 
great  danger  existed  in  defective  plumbing,  owing  to  the 
escape  of  sewer  gas  or  gases  by  leakage,  and  particularly 
from  the  pressing  backward,  or  "  rising,"  of  sewer  gas  into 
bath  rooms,  or  sleeping  rooms  provided  with  set  bowls,  etc. 
The  present  view  is  that  while  such  gases  may,  and  prob- 
ably sometimes  do,  escape  into  houses,  they  are  usually 
greatly  diluted  before  they  are  breathed  and,  at  the  worst, 
are  much  less  harmful  than  was  formerly  supposed.  They 
are,  nevertheless,  highly  objectionable,  and  it  is  likely  that 
they  occasionally  produce  serious  poisoning.  If  breathed 
for  a  long  time,  even  in  small  amounts,  they  probably 
lower  vital  resistance  and  increase  susceptibility  to  infec- 
tious disease,  and  are  thus  not  merely  objectionable  but 
also  dangerous. 

Pains  should  be  taken  to  ventilate  thoroughly  all  places, 
such  as  sleeping  rooms,  bath  rooms,  and  water-closets,  into 
which  sewer  gases  may  find  their  way,  and  it  is  advisable 
and  customary  to  seal  up  the  various  drainpipes  by  water 
seals,  or  traps.  If,  in  addition,  the  main  drainpipes  are  pro- 
vided with  vents  to  allow  the  escape  of  any  gases  accu- 
mulated in  the  pipes,  the  essentials  of  sanitary  plumbing 
have  been  secured.  Good  workmanship  is,  however,  indis- 
pensable in  all  water  and  drain  pipes,  as  well  as  in  all 
gas  pipes,  in  the  house,  to  prevent  serious  damage  from 
breaks  or  leaks. 


458  THE  HUMAN  MECHANISM 

The  main  drainpipe  in  the  house  is  called  the  soil  pipe. 
This  usually  empties  into  an  underground  drain  or  sewer 
outside  the  house,  which  discharges  its  contents,  —  now 
known  as  sewage,  —  into  a  cesspool,  or  a  stream,  or  upon  a 
sand  bed,  a  sewage  filter,  a  cultivated  field,  or  some  other 
place  of  sewage  disposal. 

10.  Drainage  and  the  Disposal  of  Household  Wastes.— 
The  consumption  of  the  solid  and  liquid  supplies  of  the 
house  —  water,  ice,  coal,  food,  etc.  —  is  accompanied  by  the 
formation  of  various  wastes  which  for  sanitary  as  well  as 
aesthetic  reasons  must  be  promptly  got  rid  of.  Waste  water 
and  melted  ice  necessitate  drainage  ;  the  dust  and  ashes  of 
fuel  remain  to  be  disposed  of,  and  from  food,  putrescible 
remnants,  known  as  garbage.  Dirt,  bottles,  papers,  boxes, 
tin  cans,  old  clothes,  worn-out  mattresses,  broken  furniture, 
crockery,  and  glass  must  also  be  removed.  Among  all  the 
wastes  of  the  house,  however,  the  discharges  of  human 
bodies  are  of  the  first  importance,  not  only  because  of  their 
putrescible  and  disagreeable  character,  but  also  because 
they  frequently  contain  the  germs  of  dangerous  diseases. 

Drainage  is  often  necessary  for  a  house  merely  to  carry 
off  rain  water  from  the  roofs,  and  to  keep  the  cellar  dry. 
It  is  very  important  to  remove  all  surplus  water  from  the 
house  and  its  vicinity  in  order  to  prevent  dampness,  —  this 
being  one  of  the  most  unfavorable  conditions  in  the  envi- 
ronment of  mankind. 

If  the  drains  of  houses  or  lands  carry  water  only,  they 
keep  the  name  of  drains,  and  the  water  in  them  is  called 
drainage;  but  if  such  drains  carry  household  wastes,  and 
especially  human  or  animal  excreta,  they  are  more  often 
called  sewers,  their  contents  being  known  as  sewage.  The 
process  or  act  of  removing  sewage  from  a  house  or  a  city, 
and  the  systems  of  sewers,  are  both  known  as  sewerage, 
although  this  same  term  is  sometimes  popularly  applied 
to  the  sewage  itself. 


WATER   SUPPLY  AND  DEAINAGE  459 

11.  The  Disposal  of  Drainage  and  Sewage. —  Cellar  drains, 
and  drains  for  the  removal  of  roof  water,  usually  discharge, 
especially  in  the  country,  upon  the  surface  of  the  ground 
at  some  distance  from  the  house,  and  give  little  trouble ; 
but  sink  drains,  since  they  contain  dish  washings,  soap- 
suds, and  the  liquid  wastes  of  the  kitchen,  are  apt  to  become 
choked  with  grease.    Grease  is  dissolved  by  alkalies,  and 
common  lye  (potash),  allowed  to  dissolve  and  flow  down 
the  sink  waste  pipe,  will  often  remove  greasy  obstructions 
and  give  at  least  temporary  relief.    The  final  disposal  of 
sink  water,  however,  is  more  difficult,  and  a  greasy,  slimy, 
malodorous,  and  unsightly  channel  or  area  behind  a  country 
house  too  often  tells  of  trouble.    The  only  complete  remedy 
is  a  large  waste  pipe,  as  straight  as  possible,  going  to  an 
equally  large  or  larger  (underground)  drain,  which  ends 
in  a  covered  pit  or  tank  placed  in  porous  or  gravelly  soil. 
This  pit  must  be  cleaned  out  from  time  to  time,  and  if 
no  open  porous  soil  is  available,  a  tight  tank  or  pit  must 
be  used  and  frequently  emptied. 

Sewage  disposal  is  a  more  difficult  matter,  for  sewage- 
contains  not  only  the  sink  wastes  just  mentioned  but  also 
washings  from  the  human  body,  human  excreta,  and  other 
putrescible  matters,  all  in  comparatively  large  volume.  We 
shall  discuss  beyond  the  problem  of  the  disposal  of  the 
mixed  sewage  of  numerous  houses  combined  into  commu- 
nities (p.  519),  and  therefore  at  this  point  need  consider 
only  the  disposal  of  the  sewage  of  separate  houses,  such  as 
country  homes  or  farmhouses.  If  these  are  so  placed  as 
to  be  readily  drained  or  sewered  into  the  sea,  or  into  some 
large  lake  or  stream  nowhere  used  for  drinking  purposes, 
the  solution  is  simple.  If  not,  the  cesspool  or  disposal 
upon  land,  as  described  in  the  next  following  paragraphs, 
are  among  the  best  expedients. 

12.  The  Cesspool  is  a  receptacle  or  tank  in  the  earth,  at 
some  distance  from  the  house,  —  not  less  than  one  hundred 


460 


THE  HUMAN  MECHANISM 


feet  away,  and  the  farther  off  the  better,  —  into  which 
sewage  is  conveyed  by  a  drain  or  sewer  directly  connected 
with  the  soil  pipe  of  the  house.  The  cesspool  may  be 
either  water-tight  to  prevent  leakage,  or  loosely  built  to 
favor  it.  A  common  construction  is  one  in  which  the  cess- 
pool is  water-tight  and  has  an  outlet  pipe  just  below  the 
surface.  This  outlet  pipe  may  run  into  a  drain,  loosely  laid 
to  facilitate  leakage  from  its  joints  and  thereby  the  escape 
of  liquid  sewage  into  the  earth. 


FIG.  117.  Disposal  of  household  sewage  by  means  of.  a  cesspool 

Observe  that  the  loosely  laid  drainpipe,  or  sewer,  allows  some  sewage  to  leach 
away  before  reaching  the  cesspool.  If  no  well  were  near,  this  would  be  a 
distinct  advantage 

It  was  formerly  held  that  the  cesspool  was  a  sanitary 
abomination ;  that  it  favored  putrefaction  and  the  develop- 
ment of  sewer  gas  ;  and  that  it  ought  always  to  be  avoided 
if  possible.  Experience  has  shown,  however,  that  thou- 
sands of  houses  have  been  drained  into  cesspools,  not  only 
conveniently  but  without  the  slightest  discoverable  sani- 
tary injury;  and  sanitary  science  now  recognizes  in  the 
septic  tank  (a  special  form  of  cesspool)  a  useful  and  popu- 
lar means  of  sewage  disposal  (Fig.  117). 


WATER  SUPPLY  AND  DRAINAGE  461 

13.  Irrigation   and  Subsurface   Disposal   of   Sewage 

Another  method  often  successful,  especially  in  soil  which 
is  open  and  porous,  i.e.  in  sandy  soil,  is  that  of  simply 
discharging  the  sewage  upon  the  surface  of  land  set  apart 
for  the  purpose  at  a  distance  from  the  house.    This  method 
may  be  recommended  in  many  cases,  but  is  often  less  sat- 
isfactory than  the  use  of  a  well-regulated  cesspool.    A  mod- 
ification of  it,  in  which  the  sewage  is  distributed  under, 
instead  of  upon,   the   surface  by  means  of  a  system  of 
branching  pipes  loosely  laid,  is  frequently  preferable,  even 
in  the  same  porous  or  sandy  soil;  but  neither  of  these 
methods  is  to  be  recommended  as  compared  with  the  cess- 
pool, if  the  soil  is  impervious  or  clayey. 

14.  The  Care   and  Disposal   of   Garbage  is  a  matter  of 
much  importance  both  to  the  housekeeper  and  to  the  com- 
munity.   Garbage  consists  chiefly  of  the  more  solid  refuse 
from  the  kitchen  and,  since  it  is  composed  of  the  remnants 
of  food,  —  bits  of  meat  and  fish,  either  cooked  or  raw,  bones, 
vegetables,  fruit,  and  the  like, — it  is  highly  putrescible.    On 
the  farm,  garbage  may  be  fed  to  swine,  and  in  many  towns 
and  cities  it  is  collected  by  farmers  and  used  to  maintain 
large  (and  often  offensive)  piggeries,  in  or  near  a  city  or 
town.    More  rarely,  garbage  is  fed  to  milch  cows,  the  milk 
from  such  cows  being  known  as  swill  milk  ;  but  this  use  of 
garbage  is  rightly  forbidden  by  boards  of  health. 

In  the  house,  garbage  is  simply  a  nuisance,  to  be  got  rid 
of  as  quickly  and  as  completely  as  possible.  It  should  be 
either  burned  (in  which  case  disagreeable  odors  are  often 
produced),  or  kept  as  short  a  time  as  possible  in  a  clean 
receptacle  in  or  near  the  kitchen.  The  garbage  recepta- 
cle is  usually  the  dirtiest  and  foulest  smelling  household 
utensil.  Nothing  about  the  house  requires  more  careful 
attention.  The  receptacle  itself  should  be  of  metal  rather 
than  wood ;  it  should  be  no  larger  than  is  necessary,  because 
a  small  can  is  easier  than  a  large  one  to  keep  clean ;  it 


462  THE  HUMAN  MECHANISM 

should  have  a  tight-fitting  cover;  and  it  should  be  kept 
where  dogs  and  cats  cannot  overturn  or  open  it.  Above 
all,  it  should  be  frequently  emptied  and  cleaned. 

15.  The  Disposal  of  Ashes,  Dirt,  and  Refuse  is  perhaps 
as  much  a  question  of  good  taste  as  of  sanitation.  Nothing 
is  more  unsightly  than  a  dump,  especially  if  its  papers, 
boxes,  and  other  combustible  materials  are  set  on  fire,  as 
often  happens,  and  left  to  smolder.  In  towns  and  cities 
ashes  and  rubbish  (as  well  as  garbage)  are  usually  removed 
periodically  by  public  carts,  but  isolated  householders  must 
ordinarily  look  after  their  refuse  disposal  themselves ;  and 
of  all  methods,  burial  in  pits,  when  possible,  is  the  least 
objectionable.  For  garbage,  especially,  when  a  piggery  is 
undesirable  and  cremation  not  possible,  burial  in  sandy 
land  at  a  distance  from  the  house  has  much  to  recom- 
mend it. 


PUBLIC   HYGIENE   AND   SANITATION 


CHAPTER  XXX 

PUBLIC  HEALTH.    INFECTIOUS  AND  CONTAGIOUS 
DISEASES.     MICROBES 

1.  The  Public  Health.  —  The  environment  of  man  con- 
sists of  two  principal,  and  very  different,  parts :  one  near, 
and  chiefly  personal,  domestic,  or  private,  including  his 
clothing,  house,  family,  and  estate ;  the  other  more  remote, 
impersonal,  and  public,  including  his  neighborhood,  village, 
town  or  city,  state,  and  country  (p.  299). 

In  sparsely  settled  districts  little  attention  is  paid  to 
any  health  beyond  that  of  the  person  or  the  family ;  and 
the  family,  as  we  have  shown  (p.  425),  is  really  a  small  and 
private  community ;  but  in  thickly  settled  regions,  such 
as  cities  and  towns,  conditions  and  problems  arise  involv- 
ing numbers  of  families,  and  public  hygiene  and  sanitation 
become  necessities.  In  the  country  each  family  generally 
has  its  own  private  water  supply,  milk  supply,  food  sup- 
ply, and  drains ;  but  in  cities  and  towns  mutual  conven- 
ience, economy,  and  safety  require  public  supplies  and 
public  drains.  Instead  of  private  roads  we  find  public 
streets ;  instead  of  private  estates,  public  parks.  Public 
gardens  and  public  markets  furnish  flowers  and  vegetables  ; 
public  conveyances,  such  as  cars,  steamboats,  and  car- 
riages, serve  public  needs ;  public  institutions  arise,  such 
as  hospitals,  schools,  almshouses,  and  jails ;  and  public 
buildings,  such  as  halls,  hotels,  churches,  schoolhouses, 
shops,  factories,  stores. 

463 


464  THE  HUMAN  MECHANISM 

In  all  such  cases  numbers,  groups,  or  masses  of  individ- 
ual families  —  called  collectively  the  people,  or  the  public  — 
are  at  times  and  as  a  whole  exposed  to  unfavorable  condi- 
tions, such  as  a  general  want  of  muscular  exercise,  lack 
of  sleep,  a  too  sedentary  life,  and  over-work ;  or  to  germs  of 
infectious  and  contagious  diseases  in  public  supplies  of 
water,  milk,  etc.  ;  or  to  foul  air,  overheating,  defective 
lighting,  gas  poisoning,  noise,  dust,  smoke,  or  impure  food ; 
some  of  which  conditions  are  chiefly  personal,  affecting 
more  or  less  directly  the  bodies  of  the  people,  while  others 
are  more  remote,  or  environmental. 

By  the  public  health  we  mean  the  health  of  the  public, 
i.e.  of  the  people  as  a  whole ;  and  the  health  of  the  pub- 
lic depends  —  just  as  the  health  of  the  individuals  who 
compose  that  public  depends  —  on  a  great  variety  of  con- 
ditions, some  of  which,  as  just  stated,  are  chiefly  internal, 
or  in  intimate  relation  with  the  persons  of  the  people,  and 
may  conveniently  be  called  hygienic;  while  others  are 
chiefly  external,  or  at  least  not  in  intimate  relation  with 
the  persons  of  the  people  but  rather  in  their  environment, 
and  may  be  described  as  sanitary  (pp.  301,  302). 

The  applications  of  the  various  branches  of  science,  such 
as  physiology,  chemistry,  bacteriology,  vital  statistics,  clima- 
tology, medicine,  engineering,  etc.,  to  the  control  of  these 
various  hygienic  and  sanitary  conditions,  and  thereby  to 
the  protection  and  promotion  of  the  public  health,  consti- 
tute the  science  of  public  health  ;  and  of  this,  as  indicated  in 
the  last  paragraph,  there  are  two  grand  divisions,  namely, 
hygiene  and  sanitation. 

2.  Public  Health  Rules  and  Regulations.  —  For  the  regu- 
lation and  control  of  those  conditions  which  are  personal  and 
domestic  we  must  look,  even  in  large  communities,  chiefly 
to  individuals  and  families;  but  even  if  individuals  and 
families  always  obeyed  the  laws  of  personal  hygiene  and 
domestic  sanitation,  the  protection  of  the  public  health  would 


INFECTIOUS  DISEASES.     MICEOBES         465 

still  require  special  supervision  and  control  of  public  sup- 
plies, public  drains,  public  vehicles,  public  institutions,  and 
the  like,  because  these  things  are  outside  and  beyond  the 
control  of  private  individuals  or  families  and  stand  in  a 
class  by  themselves.  In  point  of  fact,  however,  private 
persons  and  families  are  often  negligent  in  matters  of  this 
kind,  inflicting  damage  upon  their  neighbors  by  maintain- 
ing nuisances  of  one  kind  or  another,  or  else  by  their  care- 
lessness in  respect  to  filth  or  in  respect  to  the  spread  of 
infectious  or  contagious  diseases.  Hence  it  has  come  to 
pass  that  sanitary  and  hygienic  rules  and  regulations  have 
been  adopted  by  the  citizens  of  most  civilized  communi- 
ties for  mutual  benefit. 

3.  Public  Health  Authorities.  —  For  the  enforcement  of 
these  rules  and  regulations  (sanitary  laws)  special  public 
officials  are  usually  elected  or  appointed,  such  as  boards 
of   health,   health   officers,    city   physicians,    sanitary   in- 
spectors,   medical   inspectors,   quarantine    officers,   school 
nurses,  sanitary  police,  vaccinating  physicians,   etc.    By 
common  consent  of   the    majority  of   the  citizens  these 
officers   are   authorized  and  required  under  the  laws  to 
prepare,  publish,  and  enforce  needful  sanitary  rules  and 
regulations  for  the  protection  and  promotion  of  the  public 
health. 

4.  Public  Health  Problems.  —  In  this  and  the  following 
chapters  we  can  touch  upon  only  a  few  of  those   more 
elementary  and  important  problems  of  the  public  health  of 
which  every  educated  citizen  should  have  some  knowledge. 
Such  problems  are  almost  all  fundamentally  concerned  with 
the  control  of  infectious  diseases,  to  the  nature  of  which 
we  shall  therefore  at  once  turn  our  attention. 

Much  of  what  follows  was  formerly  the  exclusive  pos- 
session of  the  medical  profession  and  has  only  recently 
become  a  part  of  the  common  knowledge  of  mankind. 
Much  of  it  also  is  comparatively  new,  and  among  the  best 


466  THE  HUMAN  MECHANISM 

fruits  of  the  splendid  advances  of  the  last  half  century  in 
the  sciences  of  pathology,  hygiene,  and  sanitation. 

5.  Plagues,  Pestilences,  and  Epidemics  are  the  most  strik- 
ing examples  of  influences  affecting  both  personal  and 
public  health.  Only  wars,  riots,  and  great  conflagrations 
are  capable  of  throwing  communities  into  such  terror  as 
has  often  been  caused  by  plagues  or  pestilences  of  some 
swiftly  fatal  disease.  Such  was  the  plague  in  London 
described  by  Defoe  in  his  Journal  of  the  Plague  Year, 
a  story  which  has  been  well  called  "  that  truest  of  all  fic- 
tions." History  is  full  of  similar  instances.  Even  as  late 
as  1892  the  rich  and  powerful  city  of  Hamburg,  Germany, 
was  terrorized  by  a  severe  epidemic  of  Asiatic  cholera  due 
to  a  polluted  public  water  supply,  while  still  more  recently 
the  cities  of  Ithaca  in  New  York  and  Butler  in  Pennsyl- 
vania have  been  ravaged  by  the  plague  of  typhoid  fever. 

Plagues  and  pestilences  are  simply  older  names  for  great 
epidemics  of  much  dreaded  diseases,  such  as  smallpox,  yel- 
low fever,  Asiatic  cholera,  or  the  bubonic  plague,  and  the 
pesthouse  which  formerly  existed  in  many  towns  and  cities 
was  a  remote  and  isolated  shelter,  or  primitive  hospital, 
often  of  the  crudest  and  poorest  kind,  to  which  the  vic- 
tims of  pestilence  were  taken  (or  driven)  by  a  frightened 
public.  The  true  sources  of  epidemics,  plagues,  and  pesti- 
lences have  only  recently  become  known.  Savages  often 
attribute  these  to  supernatural  causes,  such  as  evil  spirits 
or  demons,  and  even  for  civilized  people  pestilences  were 
until  recently  mysterious  in  origin  and  incomprehensible 
in  behavior.  It  is  now  known,  however,  that  such  out- 
breaks are  simply  extensive  epidemics  of  contagious  or  in- 
fectious diseases,  which  may  often  be  controlled  and  even 
prevented ;  but  in  order  that  control  or  prevention  shall  be 
effective,  the  intelligent  cooperation  of  all  good  citizens  is 
essential.  It  is  one  of  our  first  duties  to  acquaint  our- 
selves with  the  nature  and  the  methods  of  prevention  of 


INFECTIOUS  DISEASES.     MICEOBES         467 

contagious  and  infectious  diseases,  and  thus  at  the  same 
time  of  plagues,  pestilences,  and  epidemics. 

6.  What  are  Infectious  and  Contagious  Diseases? — The 
discoveries  of  Pasteur,  Koch,  and  their  successors  in 
the  last  half  of  the  nineteenth  century  have  brought  to 
light  the  remarkable  fact  that  those  "fevers"  -typhoid 
fever,  malarial  fever  (malaria),  diphtheria,  smallpox,  cholera, 
tuberculosis,  etc.,  and  probably  also  measles,  chicken  pox, 
scarlet  fever,  and  many  "  colds  " —  which  attack  apparently 
healthy  persons  and  cause  a  severe  but  brief  sickness  that 
seems  to  run  its  course  and  then  cease,  are  due  to  invasions 
of  the  body  by  microparasites1  called  microbes.  Each  of 
these  contagious  or  infectious  diseases  has  its  own  special 
microbe  to  which  it  owes  its  origin  ;  and  it  is  customary 
to  speak  of  the  microbes  of  diphtheria,  of  typhoid  fever, 
of  the  bubonic  plague,  of  Asiatic  cholera,  etc.,  as  the  cause 
of  these  diseases.  Although  in  some  contagious  and  infec- 
tious diseases  the  microbe  has  not  yet  been  discovered,  all 
these  diseases  are  nevertheless  so  much  alike,  and  causa- 
tive microbes  have  been  found  in  so  many  cases,  that  all 
are  believed  to  have  a  similar  microbic  origin. 

The  view  or  theory  just  outlined  is  known  as  the  germ 
theory  of  infectious  and  contagious  diseases,  and  the  causa- 
tive microbes  are  known  as  disease  germs.  It  is  easy,  on 
this  theory,  to  see  why  these  diseases  are  "  catching."  It  is, 

1  Microparasites.  —  A  parasite  is  a  plant  or  animal  which  feeds  upon 
another  plant  or  animal  (called  its  host),  and  renders  it  no  services  in 
return.  Some  parasites,  like  fleas,  lice,  the  pork  worm  (Trichina)  and 
"  ringworm,"  are  visible  or  nearly  visible  to  the  naked  eye  ;  but  many 
others  are  invisible  and  may  be  called  microparasites.  Of  these  the  most 
important  belong  among  the  microbes  (see  p.  468)  ;  but,  as  the  microbes 
form  an  enormous  group  of  plants  and  animals,  most  of  which  are  in  no 
way  parasitic  or  harmful  to  mankind,  but  on  the  contrary  are  highly  use- 
ful, we  must  be  careful  not  to  regard  as  parasites  more  than  a  very  few  of 
the  microbes.  Those  that  do  not  lead  a  parasitic  life  are  usually  scavengers 
and  lead  a  saprophytic  life ;  that  is,  they  feed  upon  dead  organic  matters, 
often  helping  greatly  to  clean  and  to  keep  clean  the  surface  of  the  earth. 


468  THE  HUMAN  MECHANISM 

of  course,  not  the  disease  but  the  parasitic  microbes  which 
can  be  "  caught "  or  "  taken,"  as  fleas  can  be  "  taken  "  from 
a  dog,  or  bedbugs  carried  from  place  to  place  in  bedding 
or  clothing,  or  lice  "  caught "  by  children  from  lousy  play- 
mates. It  is  easy,  also,  to  understand  how  destructive  epi- 
demics, plagues,  and  pestilences  can  occur,  if  public  food 
supplies,  water  supplies,  milk  supplies,  carriages,  steamers, 
caravans,  cars,  or  other  conveyances  have  become  infected 
with  infectious  microbes  or  disease  germs. 

An  infectious  disease  is  one  in  which  the  disease  germs 
infect  (that  is,  invade)  the  body  from  without.  Such  are 
diphtheria,  typhoid  fever,  tuberculosis,  trichinosis,  scarlet 
fever,  smallpox,  measles,  chicken  pox,  and  all  the  more 
common  "  fevers."  Among  these  some  are  ordinarily  con- 
veyed quite  directly  and  quickly  from  person  to  person, 
and  to  such  infectious  diseases  the  term  "  contagious  "  is 
often  applied.  Formerly  a  sharp  line  was  drawn  between 
infection  and  contagion,  but  to-day  it  is  recognized  that 
no  such  line  exists.  Typhoid  fever,  for  example,  is  still 
sometimes  said  to  be  "  infectious  but  not  contagious."  If 
by  this  is  meant  that  it  is  not  as  often  spread  through 
the  air  or  by  mere  "  contact "  as  are  smallpox  and  some 
other  diseases,  then  it  is  true.  But  if  the  saying  means 
that  it  cannot  be  transmitted  by  mere  contact  with  the 
patient  or  his  excreta,  then  it  is  false  (see  p.  485).  It  would 
be  better  to  drop  altogether  the  term  "  contagious,"  for  its 
use  is  antiquated  and  misleading. 

7.  Microbes.  —  Brief  references  have  frequently  been 
made  on  previous  pages  to  microbes  and  their  work,  but 
we  must  now  give  them  special  consideration. 

As  the  word  implies,  microbes  (micros,  small ;  bios,  life) 
are  little  living  things,  and  they  have  been  described  as 
"  all  forms  of  life,  whether  animal  or  vegetable,  invisible  or 
barely  visible  to  the  naked  eye."  It  is  customary  to  regard 
them  as  the  smallest  of  all  living  things,  and  sometimes 


INFECTIOUS  DISEASES.     MICKOBES         469 


as  identical  with  microorganisms,  germs,  or  bacteria.  All 
bacteria,  however,  are  plants,  so  that  a  broader  term,  such  as 
"germs,"  "  microorganisms,"  or  "  microbes,"  is  required  if  the 
lowest  forms  of  animal  life  are  also  to  be  covered.  In  these 
pages  we  shall  use  the  term  microbes  for  those  forms  of  life, 
either  plant  or  animal,  which  are  invisible  or  barely  visible 


8 

I 


9 

& 


FIG.  118.  Microbes  (rod-shaped  bacteria,  or  bacilli).    (All  very 
highly  magnified) 

1,  bacilli,  some  very  large  and  some  very  small;  2,  other  forms  of  bacilli; 
3,  bacilli  forming  threads  or  filaments ;  4,  dead  or  dying  bacilli  (involu- 
tion forms') 

to  the  naked  eye,  and  of  interest  or  importance  in  physiology, 
hygiene,  and  sanitation. 

For  our  purposes  microbes  may  be  divided  into  bacteria, 
or  vegetable  microbes,  and  protozoa,  or  animal  microbes. 
The  bacteria  are  unicellular  plants  of  the  simplest  struc- 
ture and  of  three  principal  forms,  viz.  rods,  berries  or  balls, 
and  spirals.  The  rods  form  the  group  bacilli  (Fig.  118),  the 
balls  the  cocci  (pronounced  cock's  eye]  (Fig.  119),  and  the 
spirals  the  spirilla  (Fig.  120).  Bacteria  often  grow  and 
multiply  (by  simple  cell  division)  very  rapidly,  and  some  are 


470 


THE  HUMAN  MECHANISM 


capable  of  producing  within  themselves  smaller  cells,  called 
spores,  which  have  thick  walls  and  possess  great  powers 
of  resistance  (see  Fig.  121). 

The  protozoa  are  unicellular  animals,  also  of  the  simplest 
structure,  and  among  them  one  group,  the  sporozoa,  is  of 


CO 


FIG.  119.  Microbes  (ball-shaped  bacteria,  or  cocci).     (All  very 
highly  magnified) 

1,  cocci  single,  and  cocci  united  in  a  jelly  mass  or  zob'glcea;  2,  in  twos  and 
fours  (diplococci  and  tetrads) ;  3,  in  clusters  (staphylococci) ;  4,  in  chains 
or  necklaces  (streptococci) 

especial  interest  because  it  certainly  includes  the  microbes 
of  malaria,  and  probably  also  those  of  smallpox  and  of 
scarlet  fever  (see  Fig.  126). 

Microbes  are  of  interest  and  importance  to  the  physiolo- 
gist, hygienist,  and  sanitarian,  first,  because  they  are  na- 
ture's scavengers,  i.e.  removers  of  organic  waste  matters ; 
second,  because  they  are  the  ordinary  agents  of  the  decom- 
position, putrefaction,  and  decay  of  foods  and  other  valu- 
able organic  matter;  and  third,  and  especially,  because 
among  them  are  found  many  microparasites,  and  especially 
those  germs  which  cause  infectious  and  contagious  diseases 
such  as  consumption,  typhoid  fever,  diphtheria,  and  malaria. 


INFECTIOUS  DISEASES.     MICROBES 


471 


1.  Microbes  as  /Scavengers.  —  Whenever  the  dead  body  of 
a  plant  or  animal,  or  any  part  of  it,  is  left  upon  the  ground, 
or  in  water,  or  is  buried  in  the  earth,  it  soon  crumbles 
or  decays  and  disappears,  turning,  as  we  say,  to  dust  and 
ashes.  It  was  formerly  universally  believed  that  this 


FIG.  120.  Microbes  (spiral  or  screw-shaped  bacteria,  or  spirilla).    (All  very 
highly  magnified) 

1,  spirilla  from  the  human  mouth;  2,  microbes  of  Asiatic  cholera;  3,  spirilla 
of  relapsing  fever ;  4,  large  spirilla  from  ditch  water 

change  was  a  slow  combustion  or  oxidation  caused  by  the 
direct  action  of  the  oxygen  of  the  atmosphere.  It  is  now 
known,  however,  that  this  oxidation  is  due  to  the  influence 
of  microbes  which  abound  in  the  upper  layers  of  the  earth, 
and  to  a  less  extent  in  air  and  water.  These  oxidize  the 
waste  organic  matters  to  carbon  dioxide,  water,  etc.,  much 


472  THE  HUMAN  MECHANISM 

as  the  muscle  fiber  oxidizes  the  food  brought  to  it  by  the 
blood  (see  Chapter  IV). 

It  is  now  established  that  scavenging  is  one  of  the  prin- 
cipal functions  of  microbes,  for  they  abound  in  sewage, 
which  they  readily  decompose  and,  under  favorable  cir- 
cumstances, completely  purify ;  in  excrement,  which  they 
work  over  and  change  to  harmless,  inoffensive,  and  even 
useful  mineral  matters ;  and  in  many  organic  wastes,  which 
they  reduce  to  simple  and  harmless  chemical  compounds 
(Figs.  120,  4,  and  122,  4). 

2.  Microbes  as  Agents  of  Decomposition  and  Decay.  — 
The  peculiar  property  which  makes  microbes  destroyers  of 
waste  organic  matters,  and  therefore  useful  as  scavengers, 
makes  them  also  troublesome,  if  not  dangerous,  agents  of 
the  decomposition,  decay,  and  consequent  destruction  of 
useful  organic  substances,  such  as  foods.  Milk,  for  exam- 
ple, may  be  spoiled  by  lactic  acid  microbes  which  feed 
upon  its  sugar  and,  by  producing  lactic  acid  in  the  course 
of  their  feeding,  cause  the  milk  to  turn  sour ;  but,  on  the 
other  hand,  this  very  change  wrought  by  the  microbes, 
though  dreaded  by  the  milkman,  may  be  desired  by  the 
cheese-maker,  in  whose  work  the  souring  of  the  milk  is 
necessary.  The  spoiling  of  meat,  fish,  fruit,  and  many 
other  forms  of  food  is  due  almost  wholly  to  the  vital  activ- 
ity of  microbes,  and  we  have  to  use  cold  and  heat  for 
protection  against  their  inroads.  Cold,  by  chilling  and 
benumbing  them,  checks  their  growth  and  multiplication ; 
while  heat,  if  sufficiently  intense,  destroys  them  altogether. 
Upon  the  former  fact  are  based  the  important  arts  of 
refrigeration  and  cold  storage ;  upon  the  latter  the  great 
modern  industry  of  canning. 

Microbes  have  a  wide  and  useful  employment  in  the 
arts  and  industries, —  such  as  the  souring  of  milk  in  cheese- 
making,  the  flavoring  of  cheese  and  butter,  the  preparation 
of  hides  for  tanning,  the  ripening  of  manures,  the  fixation 


INFECTIOUS  DISEASES.     MICKOBES         473 


of  free  nitrogen  in  agriculture,  and  many  other  processes 
depending  upon  their  vital  activity.  But,  on  the  other 
hand,  spoiled  foods  —  especially  meat,  eggs,  and  fish  — 
may  be  not  only  disagree- 
able but  also  dangerous, 
owing  to  the  formation  by 
microbes  of  poisonous 
by-products  known  as  pto- 
maines, to  whose  agency 
have  been  attributed  severe 
outbreaks  of  acute  disease. 
Ptomaines  are  bodies  of  un- 
certain chemical  composi- 
tion, which  cause  intense 
general  prostration  and 
sometimes  death.  It  is  a 
good  rule  to  avoid  carefully 


d 


IS  1-4,  growth  and  multiplication  by  cell 
division  (fission) ;  a-g,  formation,  lib- 
eration, and  germination  of  a  spore 


FIG.  121.  Diagram  of  the  growth, 
multiplication,  and  spore  forma- 
tion of  a  bacterial  microbe 

all  meat  or  fish  which 
"tainted,"  or  suspected  of 
putrefactive  decomposition. 
At  the  same  time  there  is  reason  to  believe  that  some  out- 
breaks of  obscure  poisoning  have  been  charged  to  this 
rather  uncertain  source  largely  because  no  other  explana- 
tion could  be  as  easily  given. 

3.  Microbes  as  Disease  G-erms.  —  But  it  is  as  disease 
germs  that  microbes  are  of  the  greatest  hygienic  impor- 
tance. Long  before  the  nineteenth  century  it  had  been 
suspected  that  infectious  and  contagious  diseases  were 
caused  by  invisible  "  germs  "  of  some  kind,  but  it  was  not 
until  the  last  half  of  that  century  that  the  responsibility 
for  some  of  the'  worst  diseases  that  afflict  the  human  race 
was  clearly  and  specifically  fastened  upon  certain  microbes. 
For  this  great  discovery  we  are  indebted  chiefly  to  Louis 
Pasteur,  a  French  mineralogist  and  biologist,  and  Robert 
Koch,  a  German  physician  and  bacteriologist.  Thanks 


474 


THE  HUMAN  MECHANISM 


mainly  to  their  genius  and  their  patient  labors,  we  now 
know  that  many  of  the  infectious  and  contagious  diseases 
of  animals  and  plants  are  due  to  the  entrance  into  the 
living  body  of  specific  microbes  which,  growing  in  it  or 
upon  it,  poison  it,  and  cause  it  to  sicken  or  even  to  die. 

But  if  infectious  and  contagious  diseases  are  thus  due 
to  the  attacks  of  microbes  coming  from  the  environment, 


FIG.  122.  Some  motile  bacterial  microbes,  showing  flagella  or  cilia 
(all  very  highly  magnified) 

1,  4,  large  water  bacteria;  2,  microbes  of  Asiatic  cholera;  3,  microbes 
of  typboid  fever 

we  may  hope  to  prevent  these  diseases,  either  by  warding 
off  the  microbes  or  by  making  the  body  competent  to  re- 
sist or  overcome  them,  or  both ;  and  it  is  one  of  the  chief 
lessons  of  sanitation  and  hygiene  to  show  how  the  warding  off, 
the  resistance,  and  the  overcoming,  of  infectious  and  conta- 
gious microbes  may  be  most  effectively  accomplished. 

8.  The  Biology  and  Control  of  Microbes.  —  It  must  never 
be  forgotten  that  microbes  are  living  cells,  and  as  such 
subject  to  the  laws  which  govern  all  living  things.  As  a 
rule,  they  work  best  at  about  the  blood  heat.  They  feed 


INFECTIOUS  DISEASES.     MICROBES         475 

and  grow  and  multiply  by  cell  division.  Like  muscle  fibers 
and  gland  cells,  they  decompose  their  food  materials  and 
produce  secretions  and  by-products,  some  of  which  may 
be  harmless  and  some  harmful.  Microbes,  or  germs,  are 
killed  by  strong  acids,  strong  alkalies,  and  various  other 
substances,  which,  for  this  reason,  are  called  germicides  or 
disinfectants.  Sterilization  is  complete  removal  of  microbic 
life.  It  may  be  effected  by  germicides,  or  by  intense  heat, 
such  as  boiling  or  burning,  or  by  various  other  means. 
Pasteurization  is  incomplete  sterilization  by  heat,  at  a 
temperature  sufficient  to  destroy  most,  but  not  all,  germs. 
Antiseptics  inhibit,  but  do  not  necessarily  destroy,  microbic 
activity. 

9.  The  Prevention  of  Microbic  Diseases.  —  For  the  occur- 
rence of  any  infectious  or  contagious  disease  two  conditions 
must  be  fulfilled,  viz.  (a)  a  specific  disease  germ,  micro- 
parasite,  or  microbe,  and  (b)  a  person  susceptible  to  the 
disease  in  question.  Without  the  microbe  the  disease  can- 
not arise,  no  matter  how  favorable  for  it  the  condition  of 
the  person  may  be;  and,  on  the  other  hand,  the  microbe 
appears  to  be  powerless  to  produce  the  disease,  unless  the 
condition  of  the  person  is  favorable  for  its  reception,  life, 
and  activity.  This  being  the  case,  we  obviously  have  at 
our  command  two  principal  lines  of  defense  against  the 
attacks  of  infectious  and  contagious  disease ;  on  the  one 
hand  we  must  seek  to  obtain  (a)  control  of  disease-producing 
microbes,  and  on  the  other  to  secure  (b)  insusceptibility  or 
invulnerability  of  the  human  mechanism  to  their  attacks. 

Boards  of  health  are  constantly  seeking  to  destroy  or 
control  dangerous  microbes,  by  requiring  the  reporting  of 
all  cases  of  infectious  diseases,  by  isolation  of  such  cases, 
by  the  placarding  of  houses,  by  disinfection,  by  the  in- 
spection of  food  and  other  materials,  and  by  quarantine. 
Cities  are  purifying  their  water  supplies,  their  sewage,  and 
their  milk  supplies  with  a  view  to  warding  off  the  attacks 


476  THE  HUMAN  MECHANISM 

of  disease  germs.  Individuals  also,  if  wise,  will  take  all 
reasonable  care  to  avoid  exposure  to  infection  by  such 
germs,  and  will  endeavor,  as  far  as  is  practicable,  to  secure 
food  and  drink  free  from  microparasites  capable  of  causing 
disease. 

To  measures  of  this  kind,  devoted  to  the  destruction  or 
avoidance  of'the  active  agents  of  infectious  disease,  should 
be  added  efforts  calculated  to  lessen  personal  susceptibility 
to  their  influence  if,  as  sometimes  happens  in  spite  of  all 
precautions,  they  find  entrance  into  the  body.  The  wise 
individual  will  study  himself  and  learn  from  experience 
how  to  avoid  colds  and  other  slight  ailments;  he  will 
regulate  his  diet  and  his  exercise,  his  sleep  and  his  bath- 
ing, his  work  and  his  play,  in  order  to  build  up  and  main- 
tain abounding  health  with  which  to  meet  any  microbic 
invasions  which  may  chance  to  occur.  The  wise  and 
enlightened  community  will  also  provide  parks  and  play- 
grounds, gymnasia  and  baths,  and  other  means  which  will 
facilitate  the  cultivation  of  this  abounding  health  among 
its  citizens,  young  and  old. 


CHAPTER  XXXI 

SOME  MICROBIC  DISEASES  AND  THEIR  PREVENTION. 
VACCINATION  AND  ANTITOXIC  SERUMS 

1.  Tuberculosis.  —  One  of  the  first  diseases  of  which 
microbes  were  conclusively  proved  to  be  the  cause  was 
tuberculosis,  a  disease  to  which  more  deaths  are  annually 
attributed  than  to  any  other.  For  the  latter  reason  it  has 


|ifi"/  •-•  yV;-/    if?  '', 


u(      /      •        V7--,  ^  /'•>..-'      •>•'•    ••.•>-•':  .•.«•'-.••'  I  •-••,V^7.  ••.•:•  -:^-:.-; 

W!m     m  -•'••  -  ,     ':•> 


FIG.  123.  The    microbe    of  tuberculosis   (Bacillus  tuberculosis).     (Very 
highly  magnified) 

A  minute  amount  of  the  spit  (sputum)  of  a  consumptive  has  been  spread  out 
thin  and  photographed.  The  tubercle  bacilli  are  the  little  sticks  or  small 
rods  scattered  all  about  among  larger  irregular  epithelial  cells,  mucus,  etc. 

been  called  the  "  great  white  plague,"  and  under  the  com- 
mon name  of  "  consumption  "  one  form  of  it  is  only  too 
familiar.  The  name  "  tuberculosis  "  was  given  to  the  disease 
because  of  certain  characteristic  cheesy  nodules  or  tuber- 
cles (little  tubers)  found  in  the  lungs  and  other  tissues  of 

477 


478  THE  HUMAN  MECHANISM 

persons  affected  with  it,  and  until  1882  it  was  generally 
regarded  as  a  constitutional  disease,  readily  inherited  and 
often  "  running  in  families."  In  that  year,  however,  Koch 
announced  the  great  discovery  that  in  the  tubercles  could 
be  found  peculiar  and  apparently  characteristic  microbes 
belonging  among  the  bacteria ;  also,  that  by  special  meth- 
ods he  had  cultivated  these  microbes  pure,  or  free  from 
all  others ;  and,  finally,  that  with  them  he  had  inoculated 
healthy  guinea  pigs  and  actually  produced  tuberculosis  in 
the  infected  animals. 

Intense  interest  was  everywhere  felt  in  Koch's  splendid 
discovery,  which  was  quickly  confirmed,  and  soon  became 
an  established  and  universally  accepted  fact.  It  is  now 
known  that  tuberculosis  is  not  ordinarily  an  inherited  or 
constitutional,  but  an  acquired,  disease,  infectious,  conta- 
gious, and  environmental  in  origin,  and  due  to  the  rav- 
ages of  a  special  microparasite,  a  bacterial  microbe  named 
Bacillus  tuberculosis.  It  is  true  that  the  disease  often  runs 
in  families,  but  infectious  diseases  frequently  do  this ;  and 
the  reason  why  it  affects  some  families  so  much  more  than 
others  is  believed  to  be  simply  that  some  families  are  more 
susceptible  to  it,  more  adapted  to  its  growth,  than  others; 
precisely  as  some  soils  are  better  suited  than  others  for 
growing  wheat  or  grass  or  corn. 

2.  How  Tuberculosis  is  spread. — As  this  disease  is 
caused  by  the  invasion  of  a  specific  microbe,  it  can  only 
be  caused  by  the  entrance  of  that  microbe  into  susceptible 
bodies.  Sanitarians  generally  believe  that  the  principal 
ways  in  which  tuberculosis  is  conveyed  from  one  patient 
to  another  are  the  following  :  (a)  by  contagion,  that  is,  by 
personal  contact  of  tuberculous  with  non-tuberculous  per- 
sons, and  especially  by  kissing.  A  consumptive  mother  or 
sister  or  friend,  fondling  a  baby  and  "  smothering  it  with 
kisses,"  may  thus  transmit  the  germs  of  the  disease  to  the 
child;  (b)  by  objects  handled  or  mouthed  (such  as  food, 


MICKOBIC  DISEASES.     PREVENTION         479 

forks,  drinking  cups,  pencils,  or  towels)  first  by  consump- 
tives and  then  by  susceptible  non-consumptives ;  (c)  by  dust 
containing  sputum  expectorated  upon  streets  or  floors,  and 
then  dried  and  carried  in  the  air ;  (d)  by  milk  or  meat  of 
tuberculous  animals ;  and  (e)  by  the  moisture  of  the  breath 
thrown  off  not  as  vapor  but  as  fine  droplets  or  spray,  in 
coughing  or  even  while  talking.  It  is  not  certain  just  what 
part  any  or  all  of  these  play  in  the  dissemination  of  the 
disease.  At  one  time  it  was  thought  that  the  principal 
vehicle  of  tuberculosis  was  the  spit,  or  sputum,  of  tuber- 
culous patients  expectorated  upon  floors,  sidewalks,  or 
streets  and  afterwards  dried,  pulverized,  and  driven  about 
as  dust  particles,  which  might  readily  find  access  to  healthy 
throats  and  lungs.  There  is  no  doubt  that  the  spit  of  con- 
sumptives may,  and  often  does,  contain  the  germs  of  the 
disease,  and  for  this  reason  "  spit  cups  "  and  destructible 
handkerchiefs  (e.g.  of  paper)  should  always  be  used  by 
them ;  but  further  investigations  have  shown  that  drying 
and  exposure  to  sunlight  both  tend  to  weaken  and  to  de- 
stroy microbes,  so  that  to-day,  while  dust  is  still  regarded 
by  all  as  a  vehicle  of  very  great  importance  in  the  spread 
of  tuberculosis,  more  significance  than  formerly  is  attrib- 
uted to  other  factors. 

It  is  an  open  question  how  far  milk  is  a  carrier  of  tuber- 
culosis. Cows  undoubtedly  often  suffer  from  a  form  of 
the  disease  not  readily  distinguishable  from  that  which 
occurs  in  man,  and  the  frequency  of  tuberculosis  in  children 
certainly  suggests  that  infected  milk  may  have  been  the 
cause  of  it.  But,  on  the  other  hand,  it  is  far  from  certain 
that  bovine  tuberculosis  is  readily  transferable  to  man ; 
and  at  present,  while  it  may  be  that  milk  is  an  important 
vehicle  of  the  disease,  the  whole  question  is  much  in  doubt. 
The  same  thing  is  true  of  meat  from  tuberculous  animals, 
although  in  this  case  thorough  cooking  must  destroy  most, 
if  not  all,  of  the  germs.  The  safest  plan  is  to  use  for  food 


480  THE  HUMAN  MECHANISM 

no  milk  or  meat  which  is  in  the  least  doubtful  in  quality, 
at  least  not  without  first  subjecting  it  to  thorough  cooking. 
At  present  the  dissemination  of  the  bacilli  through  the  air 
by  means  of  finely  divided  droplets  of  mucous  or  moisture, 
as  suggested  above,  is  regarded  as  of  special  importance. 

3.  The  Prevention  of  Tuberculosis.  —  No  successful  steps 
have  yet  been  taken  toward  the  prevention  of  tuberculosis 
by  vaccination,  or  in  any  other  ways  than  by  warding  off 
the  microbe  and  by  helping  the  patient  in  his  struggle 
with  it  by  giving  him  good  air,  good  food,  rest,  and  all 
other  favorable  conditions  which  shall  aid  the  body  in 
resisting  infection  and  the  ravages  of  the  disease. 

It  has  been  proposed  to  isolate  cases  of  tuberculosis  and 
in  general  to  deal  with  them  very  much  as  the  more  con- 
tagious diseases  are  ordinarily  dealt  with.  There  is  some- 
thing to  be  said  in  favor  of  this  plan,  but  the  general 
opinion  is  that  such  isolation  is  a  hardship  to  the  patient 
and  not  often  necessary  for  the  safety  of  the  community. 
Consumptives  should,  however,  be  expected,  and  even 
required,  to  be  especially  cleanly  in  their  habits,  and  to 
collect  and  destroy  their  sputum  in  cheap  paper  cups  or 
in  paper  handkerchiefs,  which  can  readily  be  burned.  They 
should  never  spit  upon  floors  or  streets,  or  cough  into  the 
faces  of  friends  or  attendants,  and  they  should  wash  their 
hands  and  mouths  frequently  and  thoroughly. 

Milk  or  meat  derived  from  tuberculous  animals  should 
not  be  used,  and  dust,  which  may  at  any  time  contain  the 
germs  of  tuberculosis,  should  be  kept  down  as  far  as 
possible,  both  in  houses  and  in  streets.  Above  all,  every 
means  of  direct  conveyance  of  the  fresh  virulent  microbes 
from  persons  having  the  disease  to  new  victims  should 
be  carefully  avoided.  Some  of  these  means  are  kissing 
and  coughing,  by  which  latter  minute  infectious  particles 
may  be  thrown  to  a  distance  and  caught  upon  the  face  or 
hands  of  friends,  or  upon  food,  tableware,  or  linen;  and 


MICROBIC  DISEASES.     PREVENTION         481 

any  lack  of  absolute  cleanliness  in  washing  dishes,  cups, 
spoons,  napkins,  etc.,  recently  used  by  consumptives,  is  to 
be  scrupulously  avoided.  Those,  also,  who  do  the  washing 
need  to  be  on  their  guard  against  infection,  by  exercising 
extreme  care  and  cleanliness. 

There  are  many  other  forms  of  tuberculosis  besides  con- 
sumption, but  this  is  the  form  of  principal  interest  to 
students  beginning  the  study  of  hygiene  and  sanitation. 

4.  Hygiene  in  the  Treatment  of  Tuberculosis.  —  While 
consumption  is  the  cause  of  so  many  deaths,  it  is  not  neces- 
sarily a  fatal  disease.  The  Bacillus  tuberculosis  is  usually 
of  very  slow  growth  and  low  virulence ;  it  does  not,  like 
some  microbes,  produce  large  quantities  of  poisonous  tox- 
ins which,  upon  entering  the  blood,  cause  rapid  and  exten- 
sive injuries  to  most  organs.  On  the  contrary,  its  action 
is  at  first  largely  confined  to  the  spot  in  the  lungs  where 
it  has  gained  a  lodgment,  and  at  the  outset  the  constitu- 
tional disturbances  are  slight.  So  insidious  is  the  attack 
and  growth  of  the  germ  that  the  patient  does  not  at  first 
even  suspect  its  presence,  and  merely  feels  "  out  of  sorts  " 
or  urun  down."  Only  later,  when  the  pathological  processes 
have  spread  over  a  considerable  area  of  tissue,  are  the  symp- 
toms serious ;  and  frequently  the  disease  is  not  recognized 
until  irreparable  damage  is  done. 

It  is  chiefly  for  this  reason  that  consumption  claims  so 
many  victims ;  for  the  inroads  of  the  disease  are  by  no 
means  unresisted  by  the  living  cells  of  the  body.  From 
the  outset  a  struggle  between  these  cells  and  the  invading 
microbes  takes  place ;  and  it  should  be  better  known  than 
it  is  that  in  the  majority  of  cases  the  human  mechanism  is 
the  victor  in  the  struggle.  This  is  shown  by  the  fact  that 
autopsies  on  persons  who  have  died  of  other  diseases  dis- 
close in  a  surprisingly  large  percentage  of  cases  healed 
tuberculous  lesions,  where  the  presence  of  the  disease  had 
not  been  suspected.  In  other  words,  the  disease  moves  on 


482  THE  HUMAN  MECHANISM 

to  a  fatal  issue  only  when  the  vital  resistance  proves  une- 
qual to  the  defense,  and  the  mortality  from  consumption 
would  undoubtedly  be  exceedingly  low  were  sufficient 
attention  paid  to  the  hygienic  care  of  the  body  and  the 
sanitation  of  its  surroundings,  by  both  of  which  the  vital 
resistance  is  powerfully  reenforced. 

This  is  in  itself  a  powerful  argument  for  attention  to 
general  hygiene,  and  it  points  out  unmistakably  the  hygi- 
enic treatment  of  the  disease  when  once  recognized.  No 
reliance  whatever  should  be  placed  in  drugs.  On  the  con- 
trary, the  patient,  his  family,  and  friends  should  recognize 
that  the  one  hope  lies  in  the  hygienic  conduct  of  life.  The 
patient'  should  live  and  sleep  out  of  doors,  if  possible ;  he 
should  fearlessly  breathe  cold  air,  but  should  protect  the 
skin  from  chilling  by  warm  clothing ;  if  he  cannot  live  out  of 
doors,  the  windows  of  the  living  and  sleeping  room  should 
be  kept  wide  open,  even  in  winter  weather;  the  sleeping 
room  should  have  light  walls,  and  all  curtains  and  draper- 
ies which  limit  the  amount  of  sunlight  should  be  dispensed 
with;  the  furniture  of  the  room  should  be  reduced  to  a 
minimum  and  should  be  such  as  can  be  easily  cleaned ;  rest 
from  anything  but  very  moderate  muscular  activity,  and 
from  nervous  strain,  is  absolutely  essential ;  and  all  these 
measures  should  be  reenforced  by  abundant  feeding  with 
appetizing  and  easily  digested  food;  the  feeding,  indeed, 
should  be  pushed  to  the  extreme,  should  even  be  forced 
feeding,  but  only  with  easily  digestible  foods.  In  brief, 
rest,  fresh  and  cool  air,  sunshine,  and  abundant  food  are 
the  cures  for  tuberculosis,  and,  unless  the  disease  has  gone 
too  far  before  it  is  recognized,  they  are  almost  certain 
cures. 

While  some  climates  are  more  favorable  for  the  treat- 
ment of  tuberculosis  than  others,  —  a  cold,  dry  climate 
being  preferable,  —  it  should  be  understood  that  the  treat- 
ment we  have  outlined  has  been  used  with  excellent  results 


MICROBIC  DISEASES.     PREVENTION         483 

even  in  the  patient's  own  home,  wherever  that  may  be; 
and  this  hygienic  treatment  should  be  employed  whether 
or  not  it  is  possible  to  go  away  from  home. 

Continuous  high  temperature  and  high  humidity,  or 
dampness,  are  the  main  conditions  which  make  change  of 
climate  desirable ;  and  where  these  are  not  present  it  is 
generally  better  to  treat  the  patient  at  his  own  home, 
where  family  and  friends  may  not  only  minister  to  his 
needs  but  may  also  maintain  those  cheerful  surroundings 
which  count  for  so  much  both  in  sickness  and  in  health. 

5.  Typhoid  Fever.  —  This  disease  is  now  believed  to  be 
due  to  the  bacterial  microbe  known  to  bacteriologists  as 


<-=>       «=s. 


FIG.  124.  Typhoid  fever  germs  (Bacillus  typhi).   (Highly  magnified) 

Above,  swimming  free  in  normal  blood  serum  ;  below,  "  clumped  "  or  "agglu- 
tinated" in  the  serum  of  a  typhoid  fever  patient 

Bacillus  typU.  The  latter  was  observed  by  Koch,  Eberth, 
and  others  about  1879,  but  first  thoroughly  worked  out 
by  a  pupil  of  Koch,  Dr.  GafTky,  in  1884. 

Typhoid  fever  has  been  well  known  since  about  1840, 
previous  to  which  time  it  was  confused  with  typhus  (jail, 
spotted,  or  ship)  fever,  —  a  disease  now  not  often  met  with. 
It  is  one  of  the  worst  maladies  that  afflicts  mankind,  for 


484  THE  HUMAN  MECHANISM 

although  not  generally  fatal  in  more  than  about  ten  per 
cent  of  the  cases,  it  is  a  slow  fever,  disabling  the  patient 
even  when  it  does  not  kill,  and  requiring  weeks  and  often 
months  for  its  course,  and  for  convalescence.  It  is  widely 
distributed,  probably  all  over  the  world  ;  and,  although  less 
widespread  than  tuberculosis,  it  is  still  one  of  the  chief 
causes  of  death.  It  is  a  disease  which  seriously  damages 
the  intestine  and  is  one  form  of  what  is  sometimes  called 
"  inflammation  of  the  bowels."  The  microbe  finds  its  way 
into  the  alimentary  canal  with  food  or  drink,  and  is  believed 
to  multiply  in  the  intestine  and  to  invade  the  body  proper, 
producing  a  poisonous  substance  (typhotoxiri)  which  causes 
the  fever  and  otherwise  injures  the  whole  organism.  The 
germs  are  probably  cast  off  in  abundance  with  the  various 
excreta,  and  as  diarrhea  is  a  frequent  (though  not  invari- 
able) accompaniment  of  the  disease,  typhoid  fever  is  called 
a  diarrheal  disease.  At  its  beginning  and,  in  mild  attacks, 
throughout  the  whole  disease,  the  victim  may  not  be  con- 
fined to  his  bed,  but  may  "  keep  about."  Such  cases  are 
known  as  ."walking"  typhoid. 

6.  How  Typhoid  Fever  is  spread.  —  Since  the  germs  of 
this  disease  are  present  in  the  excreta,  —  both  urine  and 
faeces,  and  even  in  the  saliva,  —  it  has  been  well  called  a 
"  filth  "  disease.  The  old  idea  of  a  filth  disease,  however, 
was  that  filth  bred  disease,  and  that  almost  any  heap  of 
dirt  or  rotting  material  might  generate  disease,  especially 
typhoid  fever, -and  inflict  it  on  persons  in  the  vicinity. 
This  idea  is  now  abandoned,  for  it  is  held  that  the  germs 
of  any  one  kind  or  species  of  disease  can  come  only  from 
other  germs  of  the  same  kind;  that  is  to  say,  typhoid 
fever  can  come  only  from  a  person  or  persons  now  having, 
or  having  recently  had,  that  disease.  The  excreta  of  such 
persons  may,  nevertheless,  readily  convey  it;  and  if  food 
or  drink  are  polluted  in  any  way  by  such  excreta,  then  the 
germs  readily  find  access  to  fresh  victims. 


MICEOBIC  DISEASES.     PEEVENTION        485 

Unfortunately,  food  and  drink  are  oftener  polluted  than 
most  persons  realize.  Water  may  be  contaminated  by  sew- 
age ;  milk,  by  the  dirty  hands  of  careless  or  unclean  milkers ; 
oysters  growing  in  harbors  or  estuaries,  by  city  sewers 
discharging  therein  ;  vegetables,  by  manure ;  and  fruits 
or  berries,  by  filthy  hands.  When  we  stop  to  think  that 
filth. may  readily  find  access  to  food  and  drink  in  these  and 
many  other  ways,  it  is  clear  that  typhoid  fever  may  still  be 
called  a  filth  disease,  even  if  we  understand  by  the  term 
that  it  is  a  disease  conveyed  by  infected  filth,  but  not  bred  or 
generated  by  filth  alone. 

7.  The  Prevention  of  Typhoid  Fever.  —  This  disease  can 
be  vastly  reduced  in  amount  and  destructiveness  in  any 
community  in  which  it  abounds,  by  careful  attention  to  the 
avoidance  and  destruction  of  its  microbes,  and  by  main- 
taining high  vital  resistance  through  hygienic  and  whole- 
some living.  No  means  are  yet  known  of  vaccinating 
against  its  attacks,  as  is  done  for  smallpox,  nor  has  any 
antitoxic  serum  yet  been  discovered  effective  for  its  cure, 
as  is  antitoxin  for  diphtheria. 

The  microbes  of  typhoid  fever  may  generally  be  avoided 
by  the  use  of  pure  drinking  water,  pure  milk,  clean  vege- 
tables and  fruits,  raw  oysters  derived  only  from  harbors 
and  estuaries  free  from  sewage,  and,  in  general,  by  the  use 
of  pure  foods  and  drinks.  Microbes  are  readily  destroyed 
by  cooking  at  a  high  temperature  and,  in  the  case  of  the 
excreta  of  patients  suffering  from  typhoid,  by  disinfection, 
which,  under  the  direction  of  an  attending  physician,  should 
always  be  thoroughly  carried  out. 

It  should  never  be  forgotten  that,  contrary  to  the  gen- 
eral impression,  typhoid  fever  is  really  contagious,  i.e.  may 
be  "  taken "  by  contact,  not  necessarily  with  the  patient, 
but  very  readily  with  his  excreta,  or  with  his  linen,  or 
with  any  of  his  belongings  soiled  with  his  excreta.  Even 
trained  nurses  sometimes  seem  to  forget  this  fact,  for  not 


486  THE  HUMAN  MECHANISM 

infrequently  a  trained  nurse  contracts  the  disease  from 
her  patient.  Similar  secondary  cases  of  typhoid  fever,  espe- 
cially in  families  in  which  the  mother  or  sister  attends  the 
patient  and  at  the  same  time  prepares  food  for  the  rest  of 
the  family,  are  painfully  common. 

It  has  been  proved  again  and  again  that  persons  over- 
worked or  otherwise  in  poor  condition  are  those  most  sus- 
ceptible to  this  disease,  and  we  could  hardly  have  a  better 
instance  than  typhoid  fever  affords  of  the  double  duty 
which  a  sound  hygiene  imposes  upon  us,  namely,  (a)  the 
duty  of  avoiding  infection  and  (b)  the  duty  of  keeping 
the  mechanism  in  such  good  condition  that  if  by  accident 
infection  occurs,  the  microbes  shall  not  be  able  to  over- 
come its  vital  resistance. 

8.  Diphtheria. — This  disease  has  long  been  known  under 
the  name  of  "membranous  croup"  and  "malignant  sore 
throat,"  but  it  was  not  until  1884  that  Loeffler,  one  of  the 
pupils  of  Koch,  detected  the  microbe  now  generally  agreed 
to  be  its  sole  and  only  cause  (Fig.  125). 

It  is  believed  that  this  microbe,  Bacillus  diphtherias, 
finding  lodgment  in  the  throat  of  a  susceptible  person, 
grows  and  multiplies  there,  secreting  meanwhile  a  poison- 
ous substance,  or  toxin,  which  injures  the  tissues  of  the 
throat  and  causes  the  formation  of  the  white  spots  and  false 
membrane  so  characteristic  of  the  disease,  and  also  damages 
the  rest  of  the  body  after  its  absorption  into  the  circula- 
tion, by  producing  fever,  paralysis  of  particular  parts,  and 
sometimes  death. 

9.  How  Diphtheria  is  spread.  —  Inasmuch  as  diphtheria 
is  a  disease  of  the  throat  especially,  it  is  easy  to  see  that 
it  must  be  chiefly  conveyed  from  one  person  to  another 
by  sputum  and  by  objects  which  come  in  contact  with  the 
mouth  or  lips.    Kissing  and  fondling  among  children,  or 
parents  and  children  ;  fingers,  which,  especially  in  children, 
are  too  often  in  the  mouth;  books,  handkerchiefs,  pencils, 


MICROBIC  DISEASES.     PREVENTION 


487 


playthings,  and  food  bitten  and  passed  from  hand  to  hand, 

—all  these  may  be  vehicles  of  contagion  and  infection  in 

this  microbic  disease.    It  may  be  spread  by  dried  sputum 


- 


1  *  *     •"• 


FIG.  125.  Microbes  of  diphtheria  and  croup  (Bacillus  diphtherice). 
(Highly  magnified) 

On  the  left,  as  they  appear  under  the  microscope  in  discharges  from  the  throat 
of  a  diphtheritic  patient.  On  the  right,  after  cultivation  in  the  bacteriologi- 
cal laboratory 

in  dust,  by  milk  infected  by  milkers  suffering  with  the  dis- 
ease, by  infected  food,  and  possibly  by  pets,  such  as  cats 
and  dogs  and  even  birds,  suffering  from  the  disease. 

10.  The  Prevention  of  Diphtheria Diphtheria  being  pri- 
marily a  disease  of  the  throat,  and  therefore  distributed 
both  by  personal  contact  and  by  spittle,  it  would  seem  to 
suffice  for  its  prevention  to  isolate  patients  having  the  dis- 
ease as  long  as  they  are  capable  of  communicating  it  to 
others,  and  thus  cut  off  the  escape  and  distribution  of  the 
germs.  Unfortunately,  however,  in  this,  as  in  many  other 
infectious  diseases,  persons  often  have  the  disease  for  some 
time  before  they  or  their  friends  discover  the  fact;  and 
some  mild  cases  of  this  malady  (as  also  of  typhoid  fever 
and  other  infectious  diseases)  probably  occur  and  run  their 


488  THE  HUMAN  MECHANISM 

course  without  ever  having  revealed  their  true  character. 
Hence  arises  the  difficulty  of  accounting  for  the  origin  of 
some  apparently  inexplicable  cases,  and  also  the  difficulty 
of  stamping  out  a  disease  of  this  kind.  Diphtheria  is  not 
an  eruptive  disease,  and  ought,  therefore,  to  be  more  readily 
controlled  than  smallpox,  scarlet  fever,  or  measles,  which 
are  doubtless  most  often  disseminated  by  means  of  scales 
shed  off  from  the  skin  during  the  "  peeling  "  which  follows 
the  eruption. 

It  should  be  clearly  understood  that  all  kissing  by  per- 
sons having  sore  throats,  or  the  mouthing  of  pencils  or 
other  objects  by  children,  is  a  dangerous  practice ;  and  that 
fingers,  which  so  readily  find  their  way  into  mouths,  may 
as  easily  as  not  carry  infection  to  books,  playthings,  food, 
letters,  or  other  objects  which  are  "  handled."  Letters  sealed 
or  handled  by  diphtheritic  patients,  or  by  persons  attend- 
ing them,  have  probably  at  times  conveyed  the  germs  of 
disease  to  persons  at  a  distance  in  whom  the  appearance 
of  the  illness  seemed  quite  unaccountable.  Here  also,  as 
in  tuberculosis,  care  in  the  disposal  of  sputum  is  of  great 
importance. 

Within  a  decade  the  discovery  of  an  antitoxic  serum, 
antitoxin  (see  p.  499),  has  given  us  a  novel  and  invaluable 
means  of  defense  against  the  microbes  of  diphtheria  by 
increasing  the  resistance  of  the  human  body  so  that  it 
shall  be  no  longer  susceptible  to  the  disease.  But  as  this 
discovery  means  much  more  to  hygiene  than  the  control 
of  this  one  disease,  we  shall  devote  to  its  careful  consid- 
eration an  entire  paragraph  beyond. 

•  11.  The  Spitting  Nuisance,  as  the  habit  of  public  spitting 
is  often  called,  is  not  only  a  disgusting  nuisance  but  a  real 
menace  to  the  public  health,  because,  as  will  now  readily 
be  seen,  it  may  be  the  means  of  spreading  abroad  diseases, 
such  as  tuberculosis  and  diphtheria,  with  which  many 
persons — incipient  cases,  or  "  walking  "  cases — may  be 


MICEOBIC  DISEASES.     PREVENTION         489 

moving  about.  Fortunately,  the  habit  is  chiefly  confined 
to  one  sex,  and  this  fact  shows  how  easily  it  might  be  con- 
trolled if  custom  demanded. 

12.  Malarial  Fever,  or  "  Malaria, "  is  a  world-famous 
disease,  especially  common  in  warm  climates  but  also  fre- 
quently occurring  in  the  more  temperate  zones.  It  is  prob- 
ably by  far  the  most  important  of  all  tropical  diseases,  for 
while  it  does  not  kill  as  do  yellow  fever  and  Asiatic  chol-^ 
era,  it  is  much  more  common  and  disables  a  far  greater 
number  of  victims. 

Malaria  has  long  been  associated  in  the  popular  mind 
with  low  grounds  and  swamps.  Experience  has  shown, 
however,  that  it  cannot  be  caused  by  swamps  alone,  for 
many  swamps  and  marshes  are  entirely  free  from  malaria. 
Sometimes  it  has  seemed  to  go  with  the  digging  up  of 
earth ;  yet  the  earth  has  very  often  been  opened  and  thrown 
about  without  causing  any  malaria  whatever. 

The  true  source  of  this  disease  remained  an  absolute 
mystery  until,  in  1880,  Laveran,  a  French  investigator 
resident  in  Algiers,  discovered  in  the  blood  of  malarial 
patients  a  microbe  of  a  very  peculiar  kind,  —  not  a  bac- 
terium, nor  even  a  plant,  like  the  microbes  of  tuberculosis, 
typhoid  fever,  and  diphtheria,  but  an  animal  known  as  a 
hcematozoon,  or  sporozoon,  belonging  among  the  simplest 
animals,  or  protozoa  (see  p.  469).  Figure  126  gives  illustra- 
tions of  some  of  the  various  forms  of  the  microbe,  and  its 
life  history  is  outlined  in  the  description  of  the  figure. 

But  even  after  the  microbe  of  malaria  had  been  detected 
no  one  knew,  for  ten  years  longer,  whence  it  came,  or 
whether  it  lived  outside  of  man  at  all,  or,  if  not,  how  it  was 
conveyed  from  victim  to  victim.  When,  at  last,  in  1899,  as 
if  for  a  final  benediction  upon  mankind  by  a  century  won- 
derfully rich  in  human  and  especially  in  medical  progress, 
the  whole  subject  was  cleared  up,  the  solution  of  the  riddle 
was  found  to  be  as  simple  as  it  was  unexpected,  for  the 


FIG.  126.  The  malaria  microbe  (Plasmodium  malarice)  and  a  malarial 
mosquito.    After  Leuckart-Chun's  wall  diagram 

1,  the  microhe;  2-5,  its  growth  in  a  red  blood  corpuscle;  6,  its  multiplication 
and  escape  from  the  corpuscle  (it  is  now  ready  to  infect  fresh  blood  cor- 
puscles) ;  7,  8,  crescentic  forms  of  the  microbe.  For  further  development 
the  microbe  must  be  transferred  from  man  to  mosquito  (by  a  biting,  i.e. 
bloodsucking,  mosquito). 

hi  the  stomach  cavity  of  Anopheles :  9,  female  stage  of  the  microbe;  9a,  9&, 
male  stage ;  10,  union  of  9  with  one  of  the  vibratile  arms  of  9& ;  11 ,  the 
microbe  resulting  from  10. 

In  the  body  proper  of  Anopheles :  12,  The  microbe  (11)  has  penetrated  the 
stomach  wall  of  the  mosquito  and  embedded  itself  on  the  outer  (body)  side 
of  the  stomach.  Here  it  undergoes  a  process  of  growth,  cell  division,  and 
multiplication  (13) ,  eventually  forming  "  tumors  "  on  the  outside  (body  side) 
of  the  mosquito's  stomach,  as  shown  in  15.  From  these  tumors  the  microbes 
escape  into  the  body  cavity  of  the  mosquito  and  find  their  way  into  the 
salivary  glands  (shown  in  section  in  16).  From  these  they  are  readily  trans- 
ferred (with  the  saliva)  into  a  human  body  bitten  by  Anopheles. 

14,  female  malarial  mosquito  (head  of  male  below). 


MICKOBIC  DISEASES.     PEEVENTION         491 


vehicle  proved  to  be  a  common  insect,  a  special  mosquito, 
long  known  as  a  pest,  but  never  hitherto  suspected  or 
dreaded  as  a  bearer  of  disease.  At  once  it  became  clear 
why  malaria  is  a  disease  of  some  warm  climates  and  some 
seasons,  and  why  it  "hangs  about"  some  swamps  and  not 
about  others. 

13.  How  Malaria  is  spread.  —  The  malarial  microbe  is 
a  microparasite  (p.  467),  spending  a  part  of  its  life  in  man 
and  a  part  in  certain  mos- 
quitoes, which  are  thus  its 
"hosts."  A  mosquito  of  the 
right  kind  bites  and  sucks 
the  blood  of  a  man  having 
malaria,  and  having  thus 
become  infected  bites  other 
persons,  injecting  into  them 
germs  of  malaria,  along  with 
that  poison  which  causes  the 
familiar  swelling  often  fol- 
lowing a  mosquito  bite. 

It  is  important  to  note  that 
only  one  genus  of  mosquito 
(Anopheles),  and  that  not  the 
commonest  in  most  places, 
seems  capable  of  conveying 
the  disease.  Moreover,  it  is 
only  the  female  Anopheles  —  and  even  that  only  after  it 
has  become  infected  by  biting  a  person  having  the  disease 
-  which  can  transmit  malaria.  Hence  many  mosquitoes, 
even  if  Anopheles,  are  harmless,  as  are  all  mosquitoes  in 
regions  in  which  no  Anopheles  or  no  malarial  microbes 
exist.  For  the  causation  of  malaria  three  things  are  re- 
quired, namely,  (a)  malarial  microbes,  (b)  female  Anopheles, 
and  (c)  susceptible  victims.  Fortunately  the  first  two  do 
not  always  coexist,  and  in  case  either  is  missing  malaria 


FIG.  127.  Anopheles  punctipennis 
(female).  After  a  photograph 
from  life  by  W.  Lyman  Under- 
wood 

Common  in  the  northern  United  States 


492  THE  HUMAN  MECHANISM 

is  absent.  Many  places  abounding  in  mosquitoes  have  no 
malaria,  either  because  there  are  no  Anopheles,  or  be- 
cause the  Anopheles  pres- 
ent have  never  become  in- 
fected. 

14.  The   Prevention    of 
Malaria.  — Beyond  a  gen- 
eral reenforcement  of  the 
body  by  wholesome  living, 
the  only  means  yet  known 
of  avoiding  this  disease  is 
the  avoidance  of  mosqui- 
toes in  those    regions  in 
FIG.  128.  The  malaria  mosquito  (Anoph-  which  they  abound,  and  in 
eles)  above,  and  the  common  mos-    which  malaria  also  occurs, 
quito  (Culex)   below.    After  photo- 

graphs  from  life  by  W.  Lyman  Un-   ]  :    a    region    contains    no 
derwood  malaria,  the  mosquitoes  in 

Showing  a  characteristic  difference  in  the    it  cannot  produce  the  dis- 

resting  attitudes  Q^    If  the  region  is  ma- 

larious  but  has  no  malarial  mosquitoes,  no  fresh  cases,  so 
far  as  known,  can  occur.  But  if  malarial  fever  and  malarial 
mosquitoes  coexist,  then  the  only  hope  is  to  remove  one 
or  the  other,  and  if  possible  both.  For  relief  from  malaria 
already  fastened  upon  a  patient,  application  should  be  made 
to  a  physician.  For  the  extermination  of  mosquitoes  from 
a  neighborhood,  all  swamps  and  marshes  must  be  drained, 
and  pools  of  stagnant  water  either  treated  with  crude  petro- 
leum or  stocked  with  fishes  that  will  feed  upon  and  destroy 
the  mosquito  larvse.1 

15.  Yellow  Fever  is  an  infection  greatly  dreaded  in  the 
tropics.  It  is  now  attributed  to  a  microbe  somewhat  similar 

1  For  further  information  on  this  subject  see  Dr.  L.  0.  Howard,  Mos- 
quitoes, How  they  live,  How  they  carry  Disease,  etc.,  New  York  (McClure), 
1901 ;  W.  L.  Underwood,  Mosquitoes  and  their  Extermination,  Boston 
(W.  B.  Clarke),  1903;  Celli,  Malaria  according  to  the  New  Researches, 
New  York  (Longmans),  1900. 


MICEOBIC  DISEASES.     PREVENTION        493 

to  that  of  malaria,  and  which  is  believed  to  be  conveyed, 
like  the  malarial  microbe,  by  a  mosquito,  though  of  another 
kind,  Stegomyia  by  name.  If  this  view  is  correct,  yellow 
fever  is  to  be  prevented  only  by  avoiding  these  mosquitoes. 
Havana,  which  was  formerly  cursed  by  yellow  fever,  has 
been  virtually  freed  from  it  by 
preventive  action  based  wholly  on 
the  mosquito  theory  of  its  causa- 
tion ;  and  the  suppression  of  yellow 
fever  in  Cuban  ports,  from  which 
formerly  the  disease  was  frequently 
exported  to  the  United  States, 
means  much  to  the  southern  states 
of  the  Union. 

In  1905  a  disastrous  outbreak  _ 

FIG.  129.  The   yellow  fever 

of  yellow  fever  occurred  in  New  mosquito  (Stegomyia). 
Orleans  and  vicinity.  Its  germs  After  a  drawing  by  L.  O. 
were  probably  brought  in  from 

some  infected  district  nearer  the  tropics  by  mosquitoes 
(Stegomyia)  on  ships  and  on  fruit  (bunches  of  bananas). 
The  epidemic  was  fought  wholly  upon  the  mosquito  theory, 
and  with  results  entirely  satisfactory  to  the  sanitarians  in 
charge. 

16.  Some  Other  Infectious  or  Contagious  Diseases,  and 
how  they  are  supposed  to  be  spread.  —  In  the  case  of  some 
of  the  commonest  infectious  or  contagious  diseases  we  are 
still  in  the  dark  both  as  to  their  precise  causation  and 
the  ways  in  which  they  are  scattered  abroad.  Concern- 
ing measles,  chicken  pox,  and  whooping  cough,  for  exam- 
ple, we  are  still  awaiting  such  discoveries  as  have  already 
been  made  for  tuberculosis  and  the  other  diseases  described 
above. 

We  have  also  referred  above  (p.  380)  to  the  fact  that 
some  colds  or  influenzas  appear  to  be  infectious.  Attempts 
have  been  made  to  detect  the  microbes  of  the  "grippe" 


494 


THE  HUMAN  MECHANISM 


and  other  influenzas,  and  figures  are  often  given  of  germs 
found  associated  with  disorders  of  this  class.  Figure  130 
is  an  example  of  this  kind,  although  it  is  not  safe  to  say 
positively  that  the  microbes  chiefly  concerned  have  really 
been  as  yet  identified. 

But  while  we  patiently  wait  for  more  light,  we  have 
good  reason,  because  of  their  general  character,  to  believe 
that  these  also  are  microbic  diseases,  caused  likewise  by 
microparasites  transmitted  either  directly  or  indirectly  from 


FIG.  130.  A  so-called  "  influenza  bacillus  " 

Ou  the  left,  as  found  in  the  sputum  in  some  "  colds"  ;  on  the  right,  after 
cultivation  in  the  bacteriological  laboratory 

victim  to  victim.  Experience  has  shown  that  the  same 
kind  of  effort  which  tends  to  prevent  diseases  undoubtedly 
microbic  tends  to  prevent  these  also  ;  so  that  at  present, 
and  for  all  practical  purposes,  we  may  consider  that  we 
know  enough  about  their  causation  and  spreading  to 
enable  us  to  deal  with  them  intelligently,  fearlessly,  and 
hopefully. 

17.  Scarlet  Fever,  Measles,  and  Chicken  Pox  belong  to 
the  group  of  eruptive  diseases,  a  term  derived  from  the  fact 
that  persons  having  any  of  them  are  usually,  sooner  or 


MICKOBIC  DISEASES.     PREVENTION         495 

later,  "  broken  out,"  or  more  or  less  covered  with  an  erup- 
tion, or  rash.  When  this  eruption  heals,  scales  of  the  skin 
are  shed  off,  and  the  wide  dissemination  of  these  scales 
during  the  process  of  "  peeling  "  is  believed  to  be  one  rea- 
son why  eruptive  diseases  are  more  contagious  than  some 
microbic  diseases  (like  tuberculosis  and  diphtheria)  which 
are  not  eruptive.  Persons  suffering  from  an  eruptive  dis- 
ease should  not  be  allowed  to  go  about  among  other  people 
until  they  have  ceased  peeling. 

As  for  the  prevention  of  measles,  chicken  pox,  scar- 
let fever,  and  whooping  cough,  the  only  means  at  hand 
at  present  seem  to  be  isolation  and  non-intercourse.  To 
maintain  or  increase  the  resistance  of  the  body  no  better 
means  are  known  than  good  feeding,  temperate  living, 
and,  in  general,  a  wise  and  wholesome  conduct  of  life  ; 
yet  even  so,  immunity  is  often  purchased  only  at  the  cost 
of  one  or  more  attacks,  and  prevention  by  isolation  is 
frequently  difficult  under  the  conditions  of  modern  life, 
especially  in  tenements  and  crowded  districts. 

18.  Summer  Complaint  in  Children  is  a  severe  and  often 
dangerous  summer  diarrhea,   believed   to   be  caused   by 
microbes  and  apparently  due  in  part  to  wrong  feeding. 
In  cities  it  appears  to  be  closely  connected  with  the  use 
of  stale  milk,  and  is  often  prevented  or  overcome  by  using 
very  fresh   milk,   or  even  by  pasteurizing  ordinary  milk 
(p.  475).    This  is  readily  done  by  immersing  bottles  of 
milk  in  water  and  then  heating  the  water  to  a  tempera- 
ture of  about  160°  F.  for  five  minutes.    If  no  thermom- 
eter is  available,  it  will  suffice  to  bring  the  milk  nearly 
(but  not  quite)  to  the  boiling  point  and  keep  it  at  that 
temperature  for  a  few  minutes. 

19.  Smallpox,  a  disease  once  so  common  that  "scarce 
one  in  a  thousand "  escaped  it,  is  now  happily  rare  in 
most  highly  civilized  countries.    It  is  an  eruptive  fever, 
small  pustules  or  pocks  giving  it  its  name,  and  while  riot 


496  THE  HUMAN  MECHANISM 

as  fatal  as  some  of  the  diseases  shortly  to  be  consid- 
ered, it  is  peculiarly  loathsome  and  very  apt  to  leave  after 
it  lifelong  disfigurement,  or  pitting.  It  is  hard  to  realize 
to-day  the  dread  and  fear  with  which  our  ancestors  rightly 
regarded  smallpox,  and  for  this  reason  some  people  make 
much  more  of  the  slight  discomfort  and  insignificant  dan- 
ger of  vaccination  than  of  the  loathsome  smallpox  itself ; 
but  if  communities  ever  cease  to  take  the  simple  but  in- 
dispensable steps  required  to  prevent  smallpox,  outbreaks 
of  the  disease  will  undoubtedly  remind  them  in  alarming 
fashion  of  what  they  had  previously  escaped. 

It  is  not  yet  absolutely  proved,  although  it  is  now  gener- 
ally believed,  that  smallpox  is  caused  by  microbic  activity ; 
but  it  is  certain  that  it  is  extremely  contagious,  probably 
through  the  scales  cast  off  from  the  skin  of  those  suffer- 
ing from  it,  with  which  scales  the  specific  microbes  are 
blown  or  handed  about. 

Experience  has  shown  that  great  gain  results  from  "  iso- 
lating "  or  "  quarantining  "  smallpox  patients  in  a  deten- 
tion hospital  (to  which  the  name  of  "  pesthouse "  was 
formerly  given).  Smallpox  patients  are  thus  removed  and 
isolated,  while  those  suffering  from  typhoid  fever  or  con- 
sumption are  not,  partly  because  of  the  much  greater  con- 
tagiousness of  smallpox,  and  partly  because  of  its  graver 
and  more  loathsome  character. 

In  smallpox,  as  in  other  diseases,  wholesome  living 
diminishes  the  danger  of  infection ;  but  it  is  a  matter  of 
history  that  in  the  days  when  the  disease  was  prevalent 
and  the  question  was  put  to  the  severest  test,  general 
good  health  proved  an  unreliable  defense.  Fortunately, 
however,  the  human  race,  which  was  once  so  frightfully 
scourged  by  this  disease,  has  discovered  a  more  certain 
means  of  protection,  which  consists  not  in  the  warding 
off  or  destruction  of  microbes  but  in  an  enhancement  of 
the  powers  of  resistance  of  the  organism,  so  remarkable  as 


MICKOBIC  DISEASES.     PREVENTION        497 

to  constitute  for  extended  periods  virtual  exemption,  or 
immunity,  from  smallpox.  The  methods  by  which  this 
extraordinary  result  is  reached  are  known  as  inoculation 
and  vaccination. 

20.  Immunity.  —  The  best  of  all  defenses  against  any 
disease  would  be  complete  insusceptibility,  or  immunity, 
to  it;  for  no  matter  how  ingenious,  elaborate,  or  com- 
plete the  devices  may  be  for  preventing  disease  germs 
from  finding  access  to  the  body,  accidents  may  always 
happen  which  will  allow  them  to  enter.  Immunity,  or 
insusceptibility,  to  disease  is  therefore  one  of  the  princi- 
pal aims  of  hygiene,  one  of  the  goals  of  sanitary  science. 
Unfortunately,  natural  immunity  is  not  common,  and  arti- 
ficial immunity  is  not  easily  conferred  or  acquired,  except 
in  the  case  of  one  or  two  diseases  ;  but  there  is  good  reason 
to  hope  that  the  future  may  have  in  store  for  the  human 
race  great  gains  in  this  direction. 

Natural  immunity  means  a  natural  insusceptibility  to 
disease.  It  is  usually  constitutional  and  inherited.  The 
lower  animals,  for  example,  are  not  susceptible  to  typhoid 
fever,  and  birds  are  immune  to  anthrax,  although  mammals 
take  it  readily.  Diseases  common  to  many  species  of  ani- 
mals appear  to  be  the  exception.  In  general,  each  species 
is  immune  to  many,  if  not  most,  of  the  diseases  of  other 
species. 

By  artificial  immunity  is  meant  a  similar  exemption 
from  disease,  not  constitutional  or  inherited,  but  acquired 
in  one  way  or  another.  The  most  familiar  method  of 
becoming  immune  to  any  disease  is  to  have  the  disease  in 
question.  For  example,  long  before  inoculation  and  vaccina- 
tion were  known,  it  was  well  recognized  that  persons  who 
had  once  had  smallpox  were  not  likely  to  have  it  a  second 
time,  and  such  persons  were,  and  still  are,  in  demand  as 
nurses  for  cases  of  that  disease.  Again,  children  who  have 
had  scarlet  fever  or  measles  or  whooping  cough  are  believed 


498  THE  HUMAN  MECHANISM 

(and  rightly)  for  that  reason  to  be  less  likely  to  have  the 
same  disease  a  second  time.  There  can  be  no  question  that 
protection  is  generally  secured  in  this  way,  although  cases 
are  not  rare  in  which  such  protection,  even  if  once  secured, 
is  ultimately  lost;  for  people  sometimes  have  measles, 
typhoid  fever,  and  diphtheria  twice,  or  even  oftener. 

21.  Inoculation  for   Smallpox.  —  The    first   great   step 
towards  the  prevention  of  infectious  diseases  by  produ- 
cing artificial  immunity  from  them  was  that  of  inoculation 
for  smallpox. 

For  a  century  after  the  first  English  settlements  in  this 
country  smallpox  ravaged  Europe  and  America.  But  in 
1721  a  novel  and  ingenious  method  of  producing  immu- 
nity from  the  disease  was  introduced  into  England  from 
Constantinople,  and  quickly  reached  the  United  States. 
This  method,  known  as  inoculation,  consisted  in  'inoculat- 
ing persons  while  in  good  health  with  the  virus  of  true 
smallpox  (not  vaccine),  for  the  purpose  of  causing  them 
to  undergo  a  mild  attack  of  the  disease  while  well  and  in 
good  condition,  so  that  they  might  avoid  having  a  severe 
attack  when  unwell  and  in  poor  condition.  Inoculation  for 
smallpox  was  an  effective  preventive  and  met  with  wide 
acceptance  and  approval  both  in  England  and  in  the  United 
States.  It  was  extensively  practiced  for  nearly  a  century, 
but  was  finally  supplanted  by  the  much  safer  process  of 
vaccination,  in  which  the  inoculation  was  with  vaccine 
(the  mild  and  comparatively  harmless  virus  of  cowpox), 
instead  of  with  the  always  dangerous  smallpox  virus. 

22.  Vaccination,  one  of  the  greatest  blessings  ever  con- 
ferred upon  mankind,  was  first  invented  in  England  in 
1796  by  Edward  Jenner,  a  young  physician  of  Gloucester- 
shire.   It  consists  in  the  inoculation  of  the  human  being 
with  virus  derived  from  a  cow  having  cowpox.    A  spot, 
usually  upon  the  upper  arm,  is  scraped  by  a  lancet,  so  that 
the  outer  layers  of  the  epidermis  are  removed ;  the  spot 


MICEOBIC  DISEASES.     PREVENTION        499 

is  then  rubbed  with  an  ivory  point,  quill,  or  tube,  carry- 
ing the  virus.  A  slight  and  usually  unimportant  illness 
or  indisposition  follows,  and  the  arm  is  sore  for  a  time, 
a  characteristic  scar  remaining.  In  some  cases  the  illness 
is  more  serious ;  but  deaths  plainly  due  to  mere  vaccina- 
tion very  rarely,  if  ever,  occur. 

The  immunity  from  smallpox  produced  by  vaccination  is 
remarkable,  and  has  been  proved  over  and  over  again,  not 
only  by  the  experience  of  armies  and  nations,  but  also  by 
actual  experiment.  It  was  formerly  thought  that  "once  vac- 
cinated" was  "always  protected";  but  to-day  it  is  recog- 
nized that  occasional  revaccination  is  essential  to  complete 
immunity,  the  length  of  the  period  of  protection  usually 
fixed  nowadays  being  not  more  than  ten  years.  Indeed,  so 
variable  is  the  duration  of  the  immunity  in  different  indi- 
viduals, and  in  the  same  individual  at  different  times,  that 
the  only  safe  course  is  to  revaccinate  whenever  there  is  an 
appearance  of  smallpox  in  the  community.  It  should  also 
be  remembered  that  the  vaccination  may  fail  to  "  take  " 
merely  because  the  virus  has  been  rubbed  off  by  the 
clothing,  or  because  it  was  not  effective  to  begin  with. 
Consequently  when  any  vaccination  fails  to  "  take,"  it  is 
safest  to  try  again  a  second  or  even  a  third  time,  espe- 
cially if  we  are  unusually  exposed  to  the  contagion  of  the 
disease. 

23.  Diphtheria  Antitoxin,  and  Other  Antitoxic  Serums.  — 
As  has  been  said  above,  inoculation  against  smallpox  was 
begun  in  western  Europe  and  America  about  1721,  and 
vaccination  for  the  same  disease  at  about  the  beginning  of 
the  nineteenth  century  (1796).  No  further  progress  was 
made  in  the  art  of  immunizing  until,  in  1879,  Pasteur  suc- 
ceeded in  extending  vaccination  to  some  species  of  the 
lower  animals,  upon  which  he  conferred  immunity  from 
certain  diseases  by  using  a  modified,  or  as  he  called  it,  atten- 
uated, virus  of  the  disease  itself. 


500  THE  HUMAN  MECHANISM 

A  much  more  important  discovery  than  this  of  Pasteur 
was  made  in  1892  when  von  Behring,  a  German  bacteriolo- 
gist, found  that  the  serum  of  the  blood  of  an  animal  im- 
mune to  diphtheria  differs  from  that  of  one  not  immune, 
in  that  it  is  capable  of  neutralizing,  both  in  a  test  tube  and 
in  the  body  of  another  animal,  the  poison  (toxin)  produced 
by  diphtheria  germs.  This  great  discovery  naturally  led 
to  the  use  of  such  neutralizing,  antidotal,  or  antitoxic  serum 
(antitoxin)  in  cases  of  diphtheria  in  man,  and  such  use  of 
it  has  now  become  general. 

In  order  to  obtain  the  antitoxin,  horses  are  inoculated 
hypodermically  with  virus,  or  toxin,1  of  diphtheria  (from 
which  all  germs  have  been  removed),  at  first  in  small  doses 
but  gradually  with  larger  amounts,  until  they  have  become 
immune  to  heavy  doses.  The  serum  of  the  horse's  blood 
under  this  treatment  gradually  becomes  changed,  so  that 
it  possesses  antitoxic  or  antidotal  properties.  This  serum 
(or  antitoxin)  is  then  carefully  collected,  bottled,  and  after- 
wards used  to  cure,  and  sometimes  to  prevent,  cases  of 
diphtheria  in  human  beings. 

Von  Behring's  discovery  is  probably  one  of  the  most 
beneficent  ever  made,  because  it  has  pointed  out  the  way 
for  the  prevention  or  cure  of  other  infectious  diseases,  by 
showing  that  when  the  disease  is  due  to  a  toxin  it  may 
be  possible  in  any  case  to  produce  an  antidote  (antitoxin) 
which  shall  neutralize  and  overcome  it.  Good  progress 

1  If  the  animal  were  inoculated  with  the  germ  of  diphtheria,  instead 
of  its  toxin,  we  should  have  no  control  of  its  growth  and  the  severity  of 
the  disease  produced.  By  inoculating  with  carefully  measured  doses  of 
the  toxin,  however,  — which  does  not  increase  in  amount,  —  we  may  pro- 
duce the  symptoms  of  diphtheria  in  very  mild  form,  and  always  have  the 
course  of  the  disease  under  control.  Immunity  is  gradually  acquired 
with  but  trifling  discomfort  to  the  animal,  and  the  antitoxic  serum  is 
absolutely  free  from  the  germs  of  the  disease.  The  statement  sometimes 
made,  that  the  use  of  antitoxin  is  liable  to  produce  diphtheria  because  the 
animal  was  inoculated  with  the  germs  of  that  disease,  is  untrue,  because 
such  germs  are  never  present  in  antitoxin  properly  made. 


MICROBIC  DISEASES.     PBEVENTION         501 


has,  indeed,  been  made  already  in  this  direction  for  a 
few  other  diseases  than  diphtheria,  especially  for  tetanus 
(lockjaw). 

24.  Tetanus,  or  Lockjaw,  is  a  comparatively  rare  disease, 
although  in  America  about  the  Fourth  of  July  it  is  quite 
common  among  boys  as  a  consequence  of  accidents  attend- 
ing the  celebration  of  that  anniversary.  The  disease  is  a 


.1     / 

•V'v 


FIG.  131.  Microbes  of  lockjaw  (tetanus).    After  Kolle  and  Wassermann 

On  the  left,  in  the  ordinary  rodlike  stage  of  active  growth;   on  the  right, 
after  having  formed  resting  "  spores  "  in  the  resting  stage 

peculiar  one,  and  prolonged  muscular  contractions  or  spasms 
are  a  characteristic  symptom  of  its  advanced  stages.  These 
spasms  may  cause  the  lower  jaw  to  become  more  or  less 
set  or  fixed ;  hence  the  popular  name,  "  lockjaw." 

The  microbe  of  tetanus  is  well  known,  and  is  common  in 
the  intestine  of  herbivorous  animals  and  in  dirt  in  many 
places.  It  grows  best  in  the  absence  of  oxygen,  and  deep 
or  lacerated  wounds,  such  as  are  made  by  toy  pistols,  etc., 
appear  specially  to  favor  its  development. 

25.  Asiatic  Cholera  is  a  microbic  fever  formerly  greatly 
dreaded  all  over  the  civilized  world,  and  still  very  de- 
structive of  human  life  in  the  East,  —  for  example,  in  the 


502 


THE  HUMAX  MECHANISM 


Philippine  Islands.  The  germ  of  the  disease  was  discov- 
ered by  Koch  in  1883,  in  the  bowel  discharges  of  cholera 
patients  in  Egypt.  Cholera  appeared  in  Hamburg,  Ger- 
many, in  epidemic  form  as  late  as  1892,  causing  great 
alarm  and  about  eight  thousand  deaths.  It  is  easily  pre- 
vented by  the  same  means  used  to  prevent  typhoid  fever 


\  l'{ 


•',  l ' 


FIG.  132.  Microbes  of  Asiatic  cholera 

On  the  left,  as  they  occur  in  the  feces  of  victims  of  the  disease ;  on  the  right, 
after  cultivation  in  the  bacteriological  laboratory 

(p.  485),  and  need  no  longer  be  greatly  feared  in  any  clean 
and  well-ordered  community  supplied  with  pure  food  and 
pure  water. 

26.  The  Plague  (Bubonic  Plague)  is  the  most  famous  of 
all  the  great  epidemic  diseases  of  history.  It  has  repeatedly 
ravaged  Europe,  and  is  still  very  common  in  some  parts 
of  Asia,  such  as  India  and  China.  The  Black  Death,  which 
was  probably  a  severe  form  of  the  bubonic  plague,  was  a 
severe  epidemic  disease  of  the  fourteenth  century,  when 
it  is  said  to  have  killed  off  twenty-five  millions  of  people, 
or  from  one  fourth  to  one  half  of  the  entire  population 
of  Europe. 


MICROBIC  DISEASES.     PREVENTION         503 


The  bubonic  plague  is  believed  to  be  transmitted  through 
the  agency  of  rats  and  fleas,  —  rats,  like  human  beings, 
being  susceptible  to  the  disease.  Rats  having  the  plague 
are  supposed  to  be  bitten  by  fleas,  which  thus  become  in- 
fected and,  later,  feeding  upon  human  beings,  inoculate 
them  with  the  gerrns  of  the  disease.  In  order  to  prevent 


V 


. 

o       .0 


• 


FIG.  133.  Microbes  of  plague 

On  the  left,  as  they  occur  in  the  swollen  lymph  glands;  on  the  right,  after 
cultivation  in  the  bacteriological  laboratory 

the  plague  it  is  important,  therefore,  to  do  away  as  far  as 
possible  with  fleas  and  rats  ;  and  in  plague-stricken  districts 
preventive  measures  are  directed  chiefly  to  the  destruc- 
tion of  rats.  In  the  Philippines,  for  example,  rat  catching 
is  an  important  branch  of  sanitary  work  in  combating  the 
plague. 

27.  The  Care  of  Wounds.  —  Proper  care  is  usually  taken 
of  a  severe  wound  by  the  physician  or  surgeon  who  is  sum- 
moned to  attend  the  case.  Slight  cuts,  on  the  other  hand, 
are  frequently  neglected  as  trivial  affairs  ;  and  without' 
question  these  cuts  usually  heal  with  no  bad  after-effects, 
either  because  no  pathogenic  organisms  are  introduced,  or 
because,  if  introduced,  they  are  killed  by  one  or  another 


504  THE  HUMAN  MECHANISM 

of  the  means  of  defense  possessed  by  the  body  against 
microbic  invasion.  Infections  from  such  wounds  are,  how- 
ever, by  no  means  of  rare  occurrence,  and  it  is  safer  to 
care  for  them  whenever  possible.  The  w'ound  should  be 
washed  thoroughly  with  some  antiseptic,  —  such  as  a  solu- 
tion of  corrosive  sublimate  (1  to  1000)  or  carbolic  acid 
(1  to  40),  —  and  then  protected  by  absorbent  cotton  until 
healed.  Since  the  bacillus  of  tetanus  grows  only  in  the 
absence  of  oxygen,  it  is  generally  safer  not  to  close  the 
wound  with  anything  like  collodion,  which  entirely  pre- 
vents access  of  air.  The  bleeding  from  an  ordinary  cut 
presents  no  danger  of  undue  loss  of  blood  from  the  body, 
and,  by  washing  out  the  wound  before  clotting  takes  place, 
is  an  important  safeguard  against  infection. 


CHAPTER  XXXII 

PUBLIC  SUPPLIES  OF  FOOD,  WATER,  AND  GAS. 
PUBLIC  SEWERAGE 

1.  Communities  and  Public  Supplies.  —  Communities 
(Latin,  communis,  common)  may  be  of  neighborhoods,  vil- 
lages, towns,  or  cities,  and  are  so  called  because  they 
possess  many  things,  such  as  roads,  bridges,  post  offices, 
churches,  and  general  situation,  in  common.  Some  sup- 
plies, such  as  air,  sunlight,  and  rainfall,  are  naturally  and 
necessarily  public  and  shared  in  common.  Others,  such  as 
water,  ice,  gas,  fuel,  milk  and  other  foods,  may  or  may 
not  be  so  shared.  In  villages  some  families  keep  their 
own  cows,  while  some  buy  their  milk  from  their  neigh- 
bors, or  from  a  milkman  supplying  several  families ;  but  in 
cities  few  people  keep  cows,  while  most  people  buy  milk. 
In  villages  private  wells  abound,  but  in  large  communi- 
ties most  people  give  up  private  wells  or  springs  and  use 
a  public  water  supply. 

Public  supplies  and  public  services  are  designed  to 
meet  those  wants  or  demands  which  many  families  have 
in  common,  and  the  chief  supplies  are  those  of  water, 
ice,  gas,  and  milk.  Fuel,  transportation  facilities,  libraries, 
parks,  playgrounds,  baths,  laundries,  bakeries,  and  the 
like  are  examples  of  other  public  supplies  or  services. 
Formerly  in  rural  life  each  family  was  comparatively  inde- 
pendent, but  the  modern  town  or  city  family,  and  to  a 
great  extent  the  rural  family,  depend  upon  some  public 
supply  for  nearly  everything  that  it  has  or  uses;  for 
books,  upon  public  libraries  or  bookshops ;  for  clothing, 

505 


506  THE  HUMAN  MECHANISM 

furniture,  household  utensils,  etc.,  upon  shops  or  stores; 
for  food,  upon  markets ;  and  sometimes  even  for  housing, 
upon  hotels  or  other  public  houses. 

2.  Public    Supplies    as   Public    Conveniences  and  Safe- 
guards. —  When  such  public  supplies  or  services  are  well 
regulated,  cheap,  and  abundant  they  may  often  be  superior 
in  safety,  comfort,  and  convenience  to  private  arrangements 
for  the  same  purpose.    In  a  city  it  is  easier  and  cheaper 
to  buy  milk  than  it  is  to  keep  a  cow.    It  is  also  better  to 
do  so,  because  cows  in  cities  must  be  under  unnatural,  if 
not  unwholesome,  conditions,  and  the  milk  may  suffer  in 
quality.    It  is  more  convenient  to  draw  water  from  a  tap 
than  from  a  well,  and  city  wells  are  generally  objectionable 
because  usually  subject  to  contamination.    It  is  more  con- 
venient, more  cleanly,  and  safer  in  a  city  to  connect  a 
house  with  a  good  sewer  than  to  supply  it  with  a  privy 
and  a  sink  drain.    Public  supplies  may  thus  serve  not  only 
as  conveniences  but  also  as  sanitary  safeguards. 

3.  Public  Supplies  as  Public  Dangers.  —  On  the  other 
hand,  public  supplies  must  be  well  arranged  and  well 
regulated,  or  they  may  become  sources  of  public  danger. 
If  the  water  supply,  for  example,  is  allowed  to  become 
polluted,  a  whole  community  ma*y  be  stricken  with  typhoid 
fever  or  some  other  infectious  disease.    Hundreds  of  cases 
of  typhoid  fever  have  been  known  to  occur  among  the 
customers   of  a  single   milkman  whose  milk  supply  had 
become  infected.    Sewage-polluted  raw  oysters  have  been 
known  to  cause  the  illness  of  dozens  of  persons  at  a  single 
public  banquet. 

It  is  easy,  to  see  that  the  very  convenience  and  wide- 
spread use  of  public  supplies  which  are  not  pure,  makes 
them  doubly  dangerous.  If  a  private  supply  becomes 
polluted,  ordinarily  only  a  single  family  suffers ;  but  if  a 
public  supply  is  impure,  hundreds  or  even  thousands  of 
persons  may  perish.  The  moral  is  plain :  the  purity  of 


PUBLIC  SUPPLIES  507 

public  supplies  should  be  thoroughly  established  at  the  outset 
and  carefully  maintained.  If,  as  is  often  the  case,  public 
supplies  are  owned  or  controlled  by  the  municipality,  then 
no  persons  should  be  put  in  charge  of  them  who  are  mere 
politicians,  or  in  any  other  way  unfit  to  act  as  guardians 
of  the  public  welfare.  Expert  scientific  supervision  of  pub- 
lic supplies  is  indispensable  for  efficiency,  for  economy,  and 
for  public  safety. 

4.  Food  Supplies,  Public  and  Private.  —  The  supply  of 
foods  to  families  or  individuals  may  be  largely  from  private 
sources,  as  in  the  case  of  a  farm  upon  which  many  foods 
may  be  produced.  But  with  the  growth  of  cities  and  large 
towns  food  supplies  are  more  and  more  shared  in  com- 
mon by  many  persons  or  families ;  while  certain  necessa- 
ries or  luxuries  of  life,  such  as  fish,  sugar,  salt,  tea,  coffee, 
spices,  are,  with  rare  exceptions,  always  obtained  from  pub- 
lic supplies. 

Furthermore,  even  on  the  farm,  specialization  often 
leads  to  the  raising  of  only  one  thing  or  a  few  things, 
such  as  cotton,  corn,  or  wheat,  and  so  to  dependence  on 
public  supplies  for  other  things  which  might  be  raised  if 
it  were  worth  while.  For  example,  most  farmers  in  New 
England  might  cultivate  sugar  maples  and  make  from  the 
sap  of  their  own  trees  a  year's  supply  of  maple  sugar,  the 
purity  of  which  they  could  control ;  but  most  of  them  pre- 
fer to  raise  other  things  which  they  can  sell  or  exchange 
for  ordinary  cane  sugar,  of  the  purity  of  which  they  have 
no  knowledge.  Flour  nowadays-  is  generally  bought  in 
barrels  or  bags  taken  at  random  from  the  output  of  dis- 
tant mills  over  which  the  buyer  has  no  control.  Meat  of 
various  kinds  is  often  purchased  from  a  public  cart,  shop, 
or  market;  fish  and  shellfish,  yeast,  butter,  eggs,  cream, 
spices,  canned  and  dried  foods,  are  likewise  obtained  from 
special  dealers,  whose  stores  are  drawn  upon  by  many 
families  and  are  therefore  public  supplies.  Obviously 


508  THE  HUMAN  MECHANISM 

impurity  or  adulteration  in  any  of  these  public  supplies 
may  injure,  or  at  least  defraud,  an  entire  community. 

5.  The  Impurity  of  Foods  may  be  of  many  kinds  and 
many  degrees,  some  of  them  of  little  or  no  hygienic  signifi- 
cance.   An  excellent  spring  water,  for  instance,  may  not 
be  "  chemically  pure  "  (i.e.  containing  nothing  but  water), 
and  yet  may  be  hygienically  wholesome ;  and  milk  might 
conceivably  be  somewhat  adulterated  with  distilled  water 
without  perceptible  damage  to  the    health  of  ^the  com- 
munity. 

We  may,  for  convenience,  distinguish  three  principal 
kinds  of  impurity  in  foods :  the  first  kind,  caused  by  the 
addition  of  some  cheaper  substance,  either  already  present 
in  the  food  (as  of  water  to  milk)  or  altogether  foreign  to 
it  (as  of  sawdust  to  ground  spices).  Such  impurity  is  pro- 
duced artificially,  intentionally,  and  fraudulently,  and  is 
known  as  adulteration.  It  may  or  may  not  be  prejudicial 
to  health,  but  it  is  always  a  cheat. 

The  second  kind  of  impurity  of  foods,  known  as  their 
infection,  consists  in  the  occurrence  in  them  of  parasites 
or  microparasites,  such  impurity  being  as  a  usual  thing 
entirely  accidental  and  unintentional,  though  often  due  to 
ignorance,  negligence,  carelessness,  or  uncleanliness.  It  is 
always  prejudicial  to  the  public  health,  but  is  not  often 
due  to  a  desire  to  cheat. 

A  third  kind  of  impurity  is  that  due  to  the  use  of  unfit 
or  unclean  or  diseased  raw  materials,  disgusting  to  the  taste 
and  destructive  to  the  appetite.  This  again  arises  either 
from  negligence  or  the  desire  to  cheat. 

6.  The  Adulteration  of  Foods.  —  The    commonest  and 
most  familiar  adulteration  of  food  is  that  of  milk  by  water. 
Water  is  so  abundant  and  cheap,  and  mixes  so  readily 
with  milk,  that  it  offers  a  constant  temptation  to  dis- 
honest milkmen  who  profit  by  the  sale  of  such  milk.    It  is 
often  difficult  for  the  consumer  to  detect  this  adulteration, 


PUBLIC  SUPPLIES  509 

even  if  lie  suspects  the  cheat ;  but  it  is  easy  for  the  chemist, 
and  large  cities  should  keep  milk  inspectors  and  analysts 
constantly  on  the  watch,  in  the  interests  of  the  public  wel- 
fare. In  some  cities  the  fines  imposed  upon  dishonest  milk- 
men more  than  repay  the  cost  of  the  service. 

But  while  milk  is  the  food  whose  adulteration  is  most 
familiar,  it  is  by  no  means  the  only  adulterated  food. 
Coffee,  spices,  beverages,  sirups,  honey,  vinegar,  and  many 
other  foods  are  subject  to  serious  adulteration;  and  most 
states  and  countries  are  obliged  to  maintain  laboratories 
devoted  to  the  protection  of  the  public  against  the  adultera- 
tion of  foods  and  drugs.  Massachusetts  has  such  a  labora- 
tory in  the  Statehouse  in  Boston,  conducted  by  the  State 
Board  of  Health.  Some  of  its  revelations  are  surprising 
and  instructive.  Milk,  for  example,  is  treated  not  infre- 
quently with  artificial  coloring  materials,  and  preservatives 
such  as  formic  aldehyde,  sodium  carbonate,  and  boracic 
acid.  Of  about  one  thousand  samples  of  suspected  milk 
examined  during  July  and  August,  1898,  nearly  three  per 
cent  contained  preservatives.  Chocolate  and  cocoa  likewise 
have  been  found  to  be  frequently  adulterated  with  wheat  or 
sugar  ;  coffee  with  roasted  peas,  wheat,  pea  hulls,  chicory, 
and  sometimes  bark,  wood,  and  charcoal ;  honey  with 
cane  sugar  or  glucose;  lard  with  cotton-seed  oil;  maple 
sugar  with  other  sugars;  maple  sirup  and  molasses  with 
glucose;  pepper  with  rice  and  buckwheat;  cloves  with 
bran,  sawdust,  and  charcoal ;  mustard,  one  of  the  most 
commonly  adulterated  of  all  spices,  with  rice,  cornstarch, 
etc. ;  cider  with  salicylic  acid.  Worse  yet,  some  so-called 
"  patent  medicines  "  which  profess  to  effect  cures  contain 
the  very  substances  —  such  as  alcohol  and  morphine  - 
the  effects  of  which  they  are  supposed  to  overcome. 

7.  The  Infection  of  Foods  by  Parasites  and  Micropara- 
sites.  —  Another,  and  from  our  standpoint  much  more  im- 
portant kind  of  impurity  sometimes  occurring  in  foods 


510  THE  HUMAN  MECHANISM 

consists  in  their  infection  by  disease-producing  organisms, 
such  as  parasitic  worms  or  microbes,  —  e.g.  the  germs  of 
typhoid  fever,  scarlet  fever,  diphtheria,  etc.  Here  again 
milk  has  the  unenviable  distinction  of  serving  as  a  familiar 
example,  for  some  of  the  worst  epidemics  of  typhoid  fever 
that  have  ever  occurred  have  been  traced  conclusively  to 
the  infection  of  some  milk  supply  by  persons  suffering 
with  that  disease. 

But  milk  is  by  no  means  the  only  food  that  may  be 
rendered  impure  and  dangerous  by  infection.  Raw  hams, 
bacon,  and  sausages  may  contain  a  kind  of  almost  micro- 
scopic worm  called  Trichina,  which  sometimes  occurs  in 
the  muscles  of  hogs,  and  which,  if  not  killed  by  cook- 
ing, is  capable  of  developing  in  the  alimentary  canal  of 
man,  boring  into  the  tissues  and  encysting  in  the  muscles 
(especially  the  diaphragm),  thereby  causing  severe  dis- 
ease and  even  death.  Extensive  epidemics  of  this  disease 
(trichinosis)  have  occurred  in  Germany,  in  America,  and 
elsewhere,  due  to  ham  and  other  pork  products  (sausages) 
either  raw  or  imperfectly  cooked.  Such  foods  are  fortu- 
nately seldom  eaten  underdone  or  "  rare,"  in  America,  but 
even  here  one  should  be  careful  to  eat  them  only  when 
thoroughly  cooked,  or  well  done.  It  is  required  by  law 
that  hogs  exported  to  foreign  countries  shall  be  carefully 
examined  for  Trichina,  and  numbers  of  experts  are  con- 
stantly employed  by  the  United  States  Bureau  of  Animal 
Industry  in  making  the  necessary  microscopic  tests. 

Other  foods  subject  to  infection  and  capable  of  con- 
veying disease  are  those  which  are  either  occasionally  or 
regularly  eaten  uncooked  ;  for  example,  shellfish,  such  as 
oysters  and  clams  ;  vegetables,  such  as  celery,  parsley,  water 
cress,  lettuce,  tomatoes,  cabbage ;  and  fruits,  berries,  and 
the  like.  The  danger  lies  in  the  fact  that  they  may  have 
been  handled  by  persons  themselves  dirty  and  suffering 
from  infectious  diseases  ;  or  they  may  have  been  grown  on 


PUBLIC  SUPPLIES  511 

fields  manured  with  sewage  or  other  fecal  matters  con- 
taining germs  of  disease.  For  all  these  dangers  there  is  but 
one  sure  remedy,  namely,  sterilization  ly  cooking  at  a  high 
temperature.  But  this  in  the  nature  of  the  case  is  impos- 
sible for  many  of  the  foods  cited  above. 

Some  fruits  (oranges,  bananas,  melons,  etc.)  are  natu- 
rally protected  by  their  skins,  and  are  consequently  espe- 
cially wholesome.  Others  (such  as  cherries,  plums,  apples, 
pears)  should  be  washed  thoroughly  or  rubbed  with  a  damp 
clean  cloth  before  being  eaten.  Still  others  (grapes,  rasp- 
berries, strawberries)  may  be  immersed  in  water  and  imper- 
fectly washed,  though  they  are  seldom  really  cleaned  by  this 
process.  Moreover,  such  "  washing  "  is  apt  to  injure  the 
texture  or  flavor  of  delicate  fruits,  and  is  sometimes  avoided 
on  that  account. 

After  all  has  been  said  and  done,  preventive  measures  may 
fail' and  some  risks  must  be  taken.  The  final  defense  must 
often  come  from  that  vital  resistance,  that  good  general 
health  which  it  is  the  special  object  of  hygiene  to  secure  and 
promote.  Life  is  valuable  and  health  is  precious,  but  either 
or  both  may  be  safeguarded  at  too  great  cost.  Undue  anx- 
iety about  foods,  or  even  about  life  and  death,  is  unworthy 
of  those  who  have  at  most  but  a  few  short  years  to  live,  and 
who  in  those  few  years  have  many  better  things  to  do  than 
merely  to  keep  alive.  "  'T  is  not  the  whole  of  life  to  live." 

8.  Food  Preserving  and  Preservatives.  —  Processes  such 
as  canning,  and  cold  storage  in  wells,  cellars,  refrigerators, 
etc.,  are  of  immense  value  to  the  human  race  as  conveniences 
and  for  the  saving  of  surplus  foods.  The  packing  of  pork 
in  brine,  the  salting,  smoking,  and  drying  of  fish,  the 
"  corning "  of  beef,  and  the  pickling  of  vegetables  are 
familiar  examples  of  other  kinds  of  food  preserving.  In 
these  latter  cases  the  foods  are  saved  from  spoiling  by  sub- 
stances (brine  or  vinegar)  which  inhibit  the  growth  of  putre- 
factive microbes  and  are  therefore  called  antiseptics.  There 


512  THE  HUMAN  MECHANISM 

are  many  other  antiseptics  besides  brine  and  vinegar,  and 
chemistry  is  constantly  adding  to  the  number.  Some  of 
the  more  important  are  boracic  acid,  formaldehyde  (for- 
malin or  formol),  and  salicylic  acid. 

A  difficult  and  delicate  question  arises  when  we  come  to 
ask,  Does  the  introduction  of  chemical  antiseptics  into  foods 
make  them  impure  or  dangerous  ?  It  is  obvious  that  the  use 
of  salt  to  preserve  fish,  of  brine  for  packing  pork  or  corning 
beef,  of  smoke  for  preserving  fish,  hams,  and  dried  beef,  and 
of  vinegar  for  pickling  have  been  approved  and  sanctioned 
by  generations.  On  the  other  hand,  the  use  of  boracic  acid  or 
formalin  in  milk  is  an  undesirable  practice,  and  at  present 
the  employment  of  any  chemical  antiseptic  in  food  preserv- 
ing must  be  regarded  as  of  doubtful  justification. 

Some  food  substances  contain  acids  which  may  attack 
the  tins  in  which  they  are  put  up  for  the  market.  Blue- 
berries, for  example,  readily  corrode  tin  cans,  forming  salts 
of  tin  which  in  large  amounts  are  harmful.  The  use  of 
glass  is  therefore  preferable  for  preserved  foods  whenever 
practicable  ;  but  the  long-continued  and  very  extensive  use 
of  tin  cans  for  tomatoes,  peas,  beans,  pears,  etc.,  without 
known  harm,  indicates  that  for  many  foods  tin  cans  may 
be  used  without  much,  if  any,  danger. 

Sometimes  food  products,  such  as  peas,  clams,  etc.,  are 
chemically  treated  in  order  to  make  them  more  attractive. 
French  peas  (canned)  have  frequently  been  found  to  con- 
tain copper,  and  canned  clams  are  sometimes  bleached  to 
make  them  whiter.  It  is  needless  to  say  that  such  treat- 
ment is  almost  always  objectionable,  even  if  not  positively 
dangerous. 

The  best  preservative,  hygienically  speaking,  is  heat, 
and  this,  carefully  applied,  may  be  made  wonderfully 
effective.  The  processes  of  canning  and  preserving  are 
too  familiar  to  need  description,  but  it  is  not  always  under- 
stood that  if  the  temperature  employed  is  high  enough  and 


PUBLIC  SUPPLIES  513 

sufficiently  long-continued  it  is  of  great  hygienic  value, 
because  it  tends  to  destroy  any  disease  germs  which  may 
be  present.  It  is,  in  brief,  a  kind  of  cookery,  and  cook- 
ing tends  not  only  to  preserve  but  also  to  purify  foods. 

9.  The  Purity  of  Public  Water  Supplies.  —  Public  water 
supplies  should  be  derived  from  the  purest  possible  sources. 
Villages  and  small  cities  are  often  supplied  from  driven 
wells  or  open  basins  located  near  a  lake  or  a  river,  and 
thus  receive  ground  water  (see  p.  451).  Large  cities  and 
many  small  ones  often  secure  their  supplies  from  lakes, 
ponds,  or  rivers,  or  from  smaller  streams,  the  water  of  which 
is  stored  in  reservoirs.  Supplies  of  this  sort  are  called 
surface-water  rather  than  ground-water  supplies,  and  the 
water  from  them  is  naturally  softer  (see  p.  451). 

Ground-water  supplies  are  apt  to  be  of  good  quality  but 
limited  in  quantity.  Surface-water  supplies  are  generally 
ample  in  quantity  but  more  easily  subject  to  pollution.  For 
this  reason  they  should  not,  as  a  rule,  be  drawn  from  thickly 
inhabited  districts  or  from  rivers,  lakes,  or  smaller  streams 
into  which  sewage  or  other  polluting  matters  may  find  their 
way ;  and  they  should  never  be  drawn  from  such  sources 
unless  they  have  first  been  purified  in  some  manner. 

Some  cities,  like  Brooklyn  (New  York)  and  Lowell 
(Massachusetts),  rely  for  their  public  water  supply  in  part 
or  wholly  upon  driven  wells  ;  some,  like  Boston,  New 
York,  and  Liverpool,  upon  water  collected  in  large  reser- 
voirs from  streams  upon  comparatively  uninhabited  water- 
sheds ;  and  some,  like  Philadelphia,  Paris,  St.  Louis,  Lon- 
don, Hamburg,  Lawrence,  Albany,  upon  impure  river  water 
which  is  purified  by  filtration,  or  otherwise  treated,  before 
it  is  distributed  to  the  citizens.1 


1  The  student,  unless  already  informed,  should  familiarize  himself  with 
the  sources  and  the  possibility  of  pollution  of  the  public  water  supply,  if 
any,  of  his  own  village,  town,  or  city,  arid  should  satisfy  himself,  if  pos- 
sible, as  to  its  purity. 


514  THE  HUMAN  MECHANISM 

It  was  formerly  thought  that  running  water  sufficiently 
purified  itself,  although  as  early  as  1874  a  Royal  Commis- 
sion of  experts  on  water  supply  reported  in  England  that 
"  there  is  no  river  in  the  United  Kingdom  long  enough  to 
purify  itself  ffom  any  sewage  introduced  into  it  even  at  its 
source,"  and  the  river  Thames  is  more  than  two  hundred 
miles  long.  It  is  true  that  sewage  or  other  filth  in  streams 
often  disappears,  and  that  great  improvement  in  polluted 
streams  frequently  takes  place;  but  such  " self-purifica- 
tion" is  too  often  partial,  incomplete,  and  untrustworthy. 

In  many  cases  the  disappearance  of  obvious  pollution  is 
due  to  a  mere  dilution  of  the  filth  with  purer  water,  and 
such  dilution  may  greatly  improve  or  even  "  purify  "  it.  A 
drop  of  ink  in  a  quart  of  water  makes  a  mixture  far  less 
inky  than  the  original  drop.  On  the  other  hand,  dilution 
does  not  necessarily  mean  destruction.  A  flock  of  birds 
may  be  lost  sight  of,  but  not  destroyed,  by  scattering,  and 
the  purification  of  sewage  filth  should  mean  its  destruction 
as  such  and  its  conversion  into  harmless  substances.  Much 
true  purification  does  take  place  in  a  flowing  stream,  but 
this  is  not  usually  adequate,  and  towns  and  cities  nowa- 
days are  generally  turning  toward  filtration,  or  other  arti- 
ficial treatment  on  a  large  scale,  of  waters  which  for  any 
reason  are  suspected  of  possible  contamination.  Some  of 
these  municipal  purification  works  are  elaborate  and  costly, 
as,  for  example,  those  in  Albany,  Philadelphia,  St.  Louis, 
Ithaca  (New  York),  Lawrence  (Massachusetts),  Washington. 

10.  Public  Gas  Supplies  and  their  Dangers.  —  There  is 
no  more  danger  from  the  products  of  combustion  of  illu- 
minating gas  than  from  those  of  oil  or  other  illuminating 
materials.  The  air  of  rooms  naturally  becomes  heated  and 
more  or  less  vitiated  by  these  products,  just  as  it  does  by 
human  breath  or  any  other  waste  product  of  oxidation; 
but  illuminating  gas  properly  burned  is  no  more  danger- 
ous to  life  than  is  kerosene  oil  or  any  similar  illuminant. 


PUBLIC  SUPPLIES  515 

Unburned  gas,  on  the  other  hand,  escaping  from  pipes  or 
fixtures  is  often  extremely  dangerous,  both  because  it  is 
poisonous  and  because  in  certain  proportions  it  forms  with 
air  an  explosive  mixture. 

Illuminating  gas  is  generally  either  "natural "  gas  drawn 
ready-made  from  the  earth,  or  gas  made  from  gasoline,  oil, 
wood,  coal,  or  coal  and  water,  and  hence  known  as  "oil 
gas,"  "  wood  gas,"  "  coal  gas,"  or  "  water  gas,"  as  the  case 
may  be. 

11.  Natural  Gas  consists  chiefly  of  marsh  gas,  or  methane 
(CH4),  this  making  from  ninety  to  ninety-seven  per  cent 
of  the  whole.    It  never  contains  more  than  one  half  of 
one  per  cent  of  carbonic  oxide  (CO),  —  a  quantity  too  small 
to  do  serious  damage.    Though  irrespirable,  —  that  is,  not 
fitted  to  support  life,  —  natural  gas  is  not  poisonous.    It 
may  even  leak  into  an  apartment  in  considerable  quanti- 
ties without  endangering  life  or  seriously  damaging  health. 

12.  Coal  Gas  is  made  by  distilling  "  soft "  or  bituminous 
coal,  and  consists  chiefly  of  hydrogen  and  marsh  gas,  with 
smaller  amounts  of  carbonic  oxide  and  other  compounds  of 
carbon.    It  contains  from  six  to  ten  per  cent  of  carbonic 
oxide,  a  highly  poisonous  gas,  and  cannot  be  admitted  into 
living  or  sleeping  rooms  in  any  great  quantity  without  ex- 
treme danger  to  life.    It  also  readily  forms  explosive  mix- 
tures with  air. 

13.  Water  Gas  is  made  by  passing  steam  over  red-hot  coal 
or  coke  (carbon),  which  decomposes  the  water  vapor,  pro- 
ducing, among  other  gases,  an  abundance  of  carbon  monox- 
ide.   As  it  leaves  the  generator,  water  gas  burns  with  a 
pale  blue  flame  only.    For  lighting  purposes  it  is  therefore 
enriched  by  the  addition  of  naphtha  or  other  vapors  which 
give  it  good  illuminating  qualities.    But  even  after  this 
treatment  water  gas  generally  contains  from  twenty-five 
to  thirty  per  cent  of  carbonic  oxide  and  is  therefore  ex- 
tremely poisonous. 


516  THE  HUMAN  MECHANISM 

In  cities  supplied  with  water  gas,  cases  of  asphyxiation 
and  death  from  gas  poisoning  are  common.  These  come 
chiefly  from  ignorance  (in  blowing  out  the  gas  instead  of 
shutting  it  off)  or  carelessness  (in  turning  the  gas  on  again 
after  extinguishing  the  light),  or  from  suicidal  intent,  or 
drunkenness,  or  from  leaky  fixtures,  or  from  change  of 
pressure,  —  a  light  turned  low  being  extinguished  by  a 
decrease  of  pressure  in  the  pipes  and  the  gas  escaping  into 
the  room  afterwards  when  the  pressure  is  renewed. 

The  most  remarkable  (and  often  the  most  extensive) 
cases  of  poisoning  by  illuminating  gas  are  those  in  which 
the  inhabitants  of  houses  or  apartments  not  piped  for  gas 
have  been  poisoned  by  gas  which  has  escaped  from  a 
broken  or  leaking  main  in  an  adjoining  street.  In  these 
cases  the  gas  makes  its  way  underground  to  the  base- 
ment of  the  house  in  question,  and  then,  perhaps  partly 
robbed  of  its  warning  odors  by  passage  through  the  earth, 
rises  through  the  house  to  sicken  or  kill  those  within. 
Whole  families,  and  even  groups  of  families,  have  occa- 
sionally been  poisoned  in  this  way,  even  in  houses  or  tene- 
ments not  piped  for  gas  at  all.  The  fact  is  that  heated 
houses  act  like  chimneys  in  producing  a  strong  up-draft ; 
and  in  winter,  when  windows  and  doors  are  shut  tight,  this 
draft  sucks  in  air  from  the  surrounding  ground.  If  the 
ground  air  happens  to  be  charged  with  gas  from  a  leaky 
main,  both  air  and  gas  may  enter  the  house  and  sicken 
or  even  kill  the  inmates,  although  the  house  itself  is  not 
supposed  to  receive  any  gas.  It  has  been  estimated  that 
"  fourteen  per  cent  of  the  total  product  of  gas  plants  leaks 
into  the  streets  and  houses  of  the  cities  supplied." 

Headaches  and  malaise  (a  convenient  term  for  "  feeling 
poorly  ")  may  be  caused  by  small  and  imperceptible  leaks 
of  illuminating  gas,  and  great  care  should  be  taken  to 
have  all  gas  fitting  well  done,  and  all  leaky  joints  or  fix- 
tures made  perfectly  tight,  especially  if  the  gas  used  is 


PUBLIC  SUPPLIES  517 

water  gas,  now  very  generally  supplied  to  the  public  in 
American  cities. 

One  of  the  great  advantages  of  lighting  houses  by  elec- 
tricity is  that  it  does  away  with  all  possibility  of  gas 
poisoning  except  that  from  leaky  mains  in  public  streets, 
already  referred  to. 

The  use  of  gas  for  heating  and  cooking  requires  especial 
caution,  owing  to  the  large  quantities  used  and  the  tem- 
porary connections  often  employed  (pp.  438  and  440). 

14.  The  Purity  of  Public  Milk  Supplies.  —  Milk  is  one 
of  the  most  universal  and  most  important  of  foods.  It  is 
also  one  of  the  most  peculiar,  in  that  it  is  a  secretion  drawn 
directly  from  the  bodies  of  living  animals.  This  remark- 
able animal  secretion,  when  fresh,  is  very  sweet,  smooth, 
and  bland  to  the  taste,  but  on  exposure  to  the  air  generally 
spoils  quickly  and  sours.  It  is  obviously  not  the  air  alone 
which  causes  it  to  sour,  for  milk  is  easily  kept  sweet  a  long 
time  if  kept  in  a  cold  place,  or  if  scalded  when  it  threatens 
to  turn  sour. 

The  spoiling  and  souring  of  milk  are  caused  by  certain 
bacterial  microbes  which,  having  got  into  the  milk  as  it 
was  drawn,  or  later  from  dust,  air,  dirt,  or  unclean  pails  or 
strainers,  live  and  multiply  enormously  at  the  expense  of 
the  sugar  and  other  food  stuffs  which  milk  contains.  The 
so-called  lactic-acid  bacteria,  in  particular,  thrive  in  milk, 
especially  if  it  is  kept  warm,  and  spoil  it  by  converting 
the  milk  sugar  (lactose)  into  milk  acid  (lactic  acid). 

Milk  that  is  pure  should  be  free  from  dirt,  and  sweet 
rather  than  sour,  but  such  milk  is  unfortunately  not  always 
easy  to  obtain,  especially  in  cities.  A  black  sediment  in 
milk  indicates  dirt  (usually  cow  dung),  and  so  does  a 
"  cowy  "  taste.  Milk  may  also  be  adulterated  with  water, 
with  antiseptics,  or  with  other  substances,  as  has  been 
shown  above  (p.  509)  ;  but  the  most  serious  impurity  in 
public  milk  supplies  is  the  occurrence  of  germs  of  contagious 


518  THE  HUMAN  MECHANISM 

or  infectious  diseases.  Many  epidemics  of  typhoid  fever 
and  diphtheria  have  been  conclusively  traced  to  a  public 
milk  supply  which  served  as  the  unsuspected  vehicle  of 
the  disease.  In  all  of  these  cases  uncleanness  of  some  sort 
—  on  the  farm,  in  the  dairy,  among  the  milkmen,  or  else- 
where —  is  believed  to  have  been  always  at  the  bottom  of 
the  trouble. 

Persons  supplying  milk  to  the  public  should  take  pains 
to  keep  their  cows  healthy  and  their  cow  stables  clean;  to 
milk  only  after  careful  washing  of  the  hands,  pails,  cans, 
strainers,  etc.,  and  also  only  after  washing  the  udders  of 
the  cow ;  and  it  should  always  be  remembered  that  milk  is 
a  rich  animal  secretion  which  readily  supports  bacterial 
life  and  therefore  should  be  scrupulously  guarded  against 
any  invasion  of  dirt  or  disease.  To  secure  rich,  pure,  clean, 
and  fresh  milk  in  cities,  a  higher  price  must  probably  be 
paid  than  has  been  the  custom  hitherto.  The  demand  is 
for  better,  purer,  cleaner  milk  ;  and  for  this  it  is  reasonable 
to  expect  that  more  must  be  charged. 

It  should  also  be  remembered  that  the  number  of  bacteria 
in  milk,  unlike  that  in  water,  does  not  depend  simply  on 
the  number  that  get  in,  since  germs  multiply  very  rapidly 
in  this  rich  food  supply.  Hence  milk  as  soon  as  drawn 
should  be  chilled  as  far  as  possible  before  delivery.  The 
mere  souring  of  milk  lessens  its  digestibility,  especially 
in  the  case  of  infants,  so  that  it  is  a  matter  of  hygienic 
importance,  particularly  in  warm  weather,  to  lessen  the 
growth  of  bacteria  in  it  by  immediate  cooling  as  soon 
as  drawn  from  the  cow,  and  keeping  as  cold  as  possible 
afterwards. 

It  must  also  be  borne  in  mind  that  the  milk-producing 
industry,  while  one  of  the  oldest  known  to  man,  is  still 
largely  in  a  primitive  condition.  What  is  needed  is  a  more 
scientific  knowledge  of  the  subject,  more  intelligence,  skill, 
and  cleanliness  among  those  engaged  in  it,  and,  finally, 


PUBLIC   SUPPLIES  519 

expert  supervision  both  on  the  part  of  the  producer  and  of 
the  sanitary  authorities  of  cities. 

15.  Public  Sewerage  and  the  Disposal  of  Sewage.  —  One 
of  the  most  beneficent  procedures  in  any  community  is 
the  establishment  of  a  system  of  public  drains  which  shall 
quickly  and  effectually  remove  all  liquid  and  many  solid 
wastes,  especially  the  excreta  of  human  beings  and  other 
animals.  Well-built  sewers  not  only  do  this  but  also  carry 
off  much  "  ground  "  water,  making  the  soils  of  cities  drier 
and  therefore  .more  wholesome.  The  term  "sewerage"  is 
applied  both  to  the  act  of  draining  and  to  the  system  of 
sewers ;  the  word  "  sewage,"  to  the  contents  of  sewers. 

The  disposal  of  the  sewage  of  cities  and  towns  is  often 
a  very  serious,  difficult,  and  costly  problem.  Sometimes  the 
sewage  can  be  safely  emptied  into  a  river,  a  lake,  or  the 
sea,  but  more  often  it  is  necessary  to  purify  it,  either  upon 
land  (where  it  may  be  made  useful,  though  rarely  profit- 
able, for  agricultural  purposes),  or  by  chemical  treatment, 
or  by  microbic  (bacterial)  action  during  cesspool  or  filtra- 
tion processes.  The  problem  of  the  final  disposal  of  sew- 
age is  not  yet  fully  solved,  and  at  the  present  time  is 
engaging  the  anxious  attention  of  the  world's  ablest  sani- 
tary engineers. 


CHAPTER  XXXIII 

THE  HYGIENE  AND  SANITATION  OF  TRAVELING,  PUBLIC 
CONVEYANCES,  PUBLIC  HOUSES,  ETC. 

1.  Migration,  Past  and  Present.  —  One  of  the  most  strik- 
ing characteristics  of  the  present  as  compared  with  the 
past  is  the  increased  and  increasing  movement  of  masses 
of  people  not  only  permanently  out  of  one  country  (emigra- 
tion) and  into  another  (immigration),  but  also  temporarily 
from  place  to  place,  and  back  and  forth  (traveling).    Such 
migration  inevitably  removes  the  traveler,  temporarily  at 
least,  from  one  environment,  and  subjects  him  to  another 
and  often  very  different  one ;  so  that  from  the  hygienic 
point  of  view  a  change   of  this   sort  is  of  great  impor- 
tance and  interest.    It  also  often  affects  the  environments 
of  others  besides  the  migrant  himself,  by  introducing  into 
those  environments  new  elements  of  disease. 

2.  Traveling  and  Change  of  Scene.  —  Even  before  start- 
ing upon  a  journey  conditions  for  the  prospective  traveler 
have  often  begun  to  change.    The  bustle  and  the  thought 
of  the  necessary  preparations  constitute  a  kind  of  excite- 
ment, sometimes  pleasurable,  sometimes  wearisome,  accom- 
panied, it  may  be,  by  temporary  loss  of  appetite  or  even 
sleeplessness  (especially  in  children),  or  by  other  abnormal 
conditions   sometimes  described  by  the  phrase  "journey 
proud."    With  the  start  come  leave  takings,  farewells,  and 
partings  more  or  less  unusual  and  exciting,  and  then  begins 
a  series  of  tolerably  rapid  changes  of  environment  or  scene. 
The  body  is  moving  and  possibly  shaken  about  or  jarred ; 
unusual  and  shifting  scenes  fall  upon  the  retina  and  come 

520 


HYGIENE  OF  TBAVELIHG  521 

before  the  mind,  calling  for  attention  and  arousing  new  sen- 
sations ;  strange  sounds  are  heard,  strange  odors  detected; 
the  air  (if  in  an  open  vehicle)  beats  against  the  face  and  the 
ordinary  atmospheric  "  blanket "  is  diminished  or  otherwise 
interfered  with. 

Arrived  at  a  stopping  place  or  the  journey's  end,  streets, 
houses,  and  hotels  are  new  or  strange  ;  strange  faces  meet 
the  traveler  ;  there  are  strange  rooms  and  walls,  strange 
furnishings,  strange  sounds  and  odors,  —  in  short,  a  strange 
or  unusual  environment. 

All  this  may  or  may  not  be  wholesome,  according  to 
circumstances  on  the  one  hand  and  the  individual  on  the 
other ;  but  it  is  certainly  stimulating  and  physiologically 
exciting,  as  may  be  readily  proved  by  observing  its  effects 
upon  children  and  the  aged.  The  change  of  scene  is  only 
one  element  in  the  hygiene  of  travel,  and  its  value  must  be 
determined  by  weighing  it  together  with  other  equally  influ- 
ential factors,  namely,  the  change  of  occupation,  the  change 
of  air,  and  the  change  of  food,  and  finally  by  applying  all 
of  these  considerations  to  particular  cases  or  individuals. 

3.  The  Change  of  Occupation.  —  It  is  an  old  saying  that 
"  all  work  and  no  play  makes  Jack  a  dull  boy,"  and  expe- 
rience teaches  clearly  enough  that  a  change  of  occupation 
is  wholesome.  One  of  the  best  features  of  travel  is  that 
it  necessitates  a  change  of  occupation.  A  common  expres- 
sion contains  the  idea  of  "  going  away  from  home  to  get 
a  change."  One  of  the  most  valuable  characteristics  of  the 
home  is  the  repose  and  restfulness  which  result  from  its 
uniformity  of  conditions,  and  one  of  the  best  things  about 
travel  is  the  mild  stir  and  excitement  involved.  Routine 
and  regularity  of  occupation  are  on  the  whole  the  more 
natural  and  normal,  and  "  a  steady  job,"  whether  it  be  in 
shop,  mine,  or  factory ;  on  farm,  plantation,  or  shipboard ; 
in  bank,  school,  or  professional  life,  is  naturally  sought  and 
prized  by  everybody. 


522  THE  HUMAN  MECHANISM 

And  yet  most  persons  profit  from  time  to  time  by  "a 
day  off,"  or  a  vacation,  or  a  journey  which  affords  change 
of  occupation  with  freedom  from  responsibility.  Once  on 
the  way,  the  traveler  is  not  responsible  for  the  train  or  the 
steamer,  for  the  cookery  or  the  beds,  for  the  house  or  hotel, 
or  its  furnishings  or  management ;  and  this  freedom  from 
responsibility  is  a  complete  and  often  refreshing  change. 

4.  The  Change  of  Air.  —  It  is  difficult  to  say  in  what 
way  and  to  what  extent  a  change  of  air  is  beneficial  in 
traveling.  Much  of  the  benefit,  even  when  attributed  to 
the  "  change  of  air,"  is  no  doubt  really  due  to  other  things, 
such  as  the  change  of  work  and  the  change  of  scene ;  but 
after  making  all  allowances,  it  would  still  seem  to  be  true 
that  a  change  of  air  has  a  perceptible  effect,  and  often  does 
great  good  or  great  harm.  Air  that  is  drier  or  damper, 
or  warmer  or  cooler  than  usual,  or  air  in  the  forest  or  by 
the  sea,  often  seems  to  have  decided  effects  for  good  or  for 
evil,  all  other  conditions  remaining  apparently  much  the 
same.  At  times,  obscure  atmospheric  influences  at  home, 
unknown  to  ourselves,  may  be  the  source  of  lessened  vital 
resistance,  and  so  of  a  lowered  tone  of  general  health,  and 
the  change  of  air  may  be  the  means  of  restoring  nor- 
mal conditions  by  removing  the  obscure  cause  of  trouble. 
Moreover,  when  the  change  is  from  the  close,  "stuffy"  air 
of  an  office  to  the  open  air  of  country  or  of  sea,  with  their 
agreeable  odors,  there  is  a  "  bracing  "  or  stimulating  effect 
which  reacts  favorably  upon  the  entire  constitution,  but 
especially  upon  the  nervous  system.  The  tendency  to  "fill 
our  lungs  "  with  it  is  only  a  sign  of  the  general  beneficial 
influence  upon  the  system  as  a  whole.  Many  a  case  of 
"  the  blues  "  has  been  successfully  overcome  by  this  simple 
expedient. 

On  the  other  hand,  air  as  a  vehicle  of  infection  may 
affect  the  traveler  unfavorably;  for  he  must  almost  inevi- 
tably be  exposed  to  air  (as  well  as  to  other  things)  which  has 


HYGIENE  OF  TBAVELING  523 

recently  been  in  contact  with  persons  having  incipient  tuber- 
culosis, diphtheria,  measles,  typhoid  fever,  or  other  infec- 
tious diseases.  To  this  subject  we  shall  return  in  the  next 
section  but  one. 

5.  The  Change  of  Food.  — It  is  uncertain  how  much  or 
how  little  influence  a  change  of  food  has  upon  the  organ- 
ism.   It  is  commonly  believed  that  a  change  of  food  is 
often  beneficial,  or  the  reverse,  and  that  much  of  the  good 
or  bad  effects  of  travel  is  due  to  the  inevitable  change  of 
diet  and  of  cookery  which  goes  with  it.    How  far  this  is 
true  is  unknown,  but  it  is  easy  to  see  that  a  simpler  diet 
for  some  and   a  more  abundant  diet  for  others  may  in 
itself  alone  be  helpful.    It  is  doubtful  if  any  special  virtue 
resides  in  "  sea  food,"  or  in  "  country  living,"  or  in  "  camp 
cookery,"  apart  from  that  which  consists  in  its  palatability 
or  its  novelty,  —  qualities  which  affect  appetites  and  there- 
fore nutrition ;  but  in  so  far  as  a  change  makes  food  appe- 
tizing or  acceptable,  such  food  is,  of  course,  more  valuable 
to  the  body.    However  this  may  be,  there  can  be  no  ques- 
tion about  the  increased  danger  of  infection  from  food  and 
drink  taken  at  random  from  unknown  sources. 

6.  The  Dangers  of  Infection  away  from  Home.  —  At  home 
the  traveler,  in  theory  at  least,  has  itn  environment  well 
under  his  control ;  but  when  he  starts  upon  a  journey,  — 
whether  afoot,  or  riding,  or  driving  ;  by  bicycle,  railway, 
steamship,  or  other  means  of  conveyance,  —  he  enters  into 
new  environments,  of  whose  precise  nature  he  is  ignorant, 
and  which  are  usually  beyond  his  control.     Of  the  san- 
itary  or  unsanitary   condition  of   the  water  supply,   ice 
supply,  milk  supply,  etc.,  he  is  entirely  ignorant;  and  he 
may  at  any  time  be  thrown  in  contact  with  persons  suf- 
fering from  infectious   diseases,   especially  in  a  mild  or 
incipient  form.    The  public  vehicle  (carriage,  wagon,  car, 
or  omnibus)  in  which  he  travels,  the  hotels,  rooms,  chairs, 
and  even  the  beds  which  he  uses  may  have  been  recently 


524  THE  HUMAN  MECHANISM 

occupied  by  diseased  persons.  His  laundry  work  may  be 
done  or  delivered  by  workers  suffering  from  contagious 
diseases  ;  uncleanness  may  attend  the  preparation  and  serv- 
ing of  his  food.  In  short,  in  leaving  his  own  familiar 
and  controllable  environment  and  passing  into  others  unfa- 
miliar and  beyond  his  control,  the  traveler  clearly  takes 
large  risks. 

7.  Safeguards  of  the  Traveler.  —  If  it  be  asked  what 
one  can  do  to  protect  himself  or  his  family  from  the  dan- 
gers of  travel,  it  may  be  pointed  out,  in  the  first  place,  that 
it  is  often  better  not  to  travel  at  all.  When  one  is  in  poor 
condition,  although  a  change  to  some  new  scene  whose 
hygienic  conditions  are  known  to  be  good  is  likely  to  be 
beneficial,  a  railroad  journey  with  frequent  stops  is  apt  to 
increase  the  danger  of  infection  at  a  time  when  vital  resist- 
ance is  low.  When  a  journey  is  necessary  the  traveler 
should  try  to  avoid  marked  fatigue,  which  always  dimin- 
ishes vital  resistance  and  thus  predisposes  to  disease ;  he 
should  seek  to  avoid  unclean  hotels,  unclean  conveyances, 
badly  aired  rooms,  and  unclean  fellow-travelers ;  he  should 
avoid  the  use  of  public  drinking  cups,  public  towels,  pub- 
lic razors,  and  the  like ;  he  should,  if  possible,  drink  only 
waters  of  established  reputation  ;  he  can,  if  need  be,  forego 
the  use  of  ra\.  milk,  raw  oysters,  and  other  uncooked 
foods  the  antecedents  of  which  he  knows  nothing  about, 
and  he  can  taVe  other  obvious  and  useful  precautions  that 
will  suggest  themselves  as  he  goes  along. 

But,  after  all,  it  must  be  admitted  that  precautions, 
even  if  rigorously  observed,  will  often  prove  insufficient, 
and  also  that  too  much  thought  about  them,  or  about  the 
dangers  of  travel,  would  rob  it  of  most  of  its  advantages. 
People  always  have  traveled  and  probably  always  will 
travel  without  much  consideration  of  the  dangers  involved 
in  traveling.  Some  risks  must  always  be  taken,  even  at 
home,  and  most  travelers  cheerfully  accept  the  necessary 


HYGIENE  OF  TRAVELING  525 

risks  for  the  sake  of  the  gains  to  be  derived.  With  the 
increase  in  the  amount  of  traveling,  many  of  the  risks 
are  gradually  decreasing,  and  in  highly  civilized  countries 
adults  in  robust  health  who  know  how  to  take  care  of 
themselves  may  now  go  upon  a  journey  without  veiy  much 
more  risk  of  infection  than  they  would  undergo  if  they 
stayed  at  home.  This  is  probably  less  true  of  children,  for 
children  and  old  people  are  not  only  more  easily  excited 
and  more  easily  fatigued,  but  they  also  suffer  more  severely 
from  exposure,  and  children  are  especially  apt  to  contract 
infectious  diseases  when  away  from  home. 

8.  Public  Drinking  Cups  should  be  avoided  by  travel- 
ers, theater  goers,  and  all  persons  in  parks  or  other  public 
places.    Few  sights  are   more  distressing  to  a  sanitarian 
than  to  see  (on  a  hot  day  in  a  crowded  railway  car)  men, 
women,  and  children,  of  all  ages,  sorts,   and  conditions, 
clean  and  unclean,  sick  and  well,  one   after   another  in 
rapid  succession  applying  their  mouths  to  the  one  public 
drinking  cup.    If  the  student  will  once  carefully  observe 
for  himself  the  use  to  which  this  cup  is  put  during  even  a 
short  journey  under  such  conditions,  he  will  realize  that 
every  traveler  had  better  carry  his  own  drinking  cup,  or, 
in  default  of  this,  go  thirsty.    In  some  theaters,  between 
the  acts,  trays  containing  glasses  of  water  are  passed  to 
patrons  in  their  seats.    Here  also  the  lips  of  many  persons 
touch  successively  the  same  glasses,  and  one  who  is  wise 
will  avoid  the  obvious  danger  involved  in  using  one  of 
these   glasses,   which    may   have   become    infected.    Sani- 
tary drinking  fountains  in  which,  by  a  simple  device,  the 
obnoxious  common  drinking  cup  is  made  unnecessary,  are 
now  being  gradually   introduced  in  parks,   schools,  and 
other  public  places. 

9.  The  Influence  of  Travelers  upon  the  Environment.  — 
We  have  thus  far  considered  chiefly  the  effects  of  strange 
environments  upon  the  traveler,  but  before  leaving  the 


526  THE  HUMAN  MECHANISM 

subject  we  must  not  fail  to  point  out  some  of  the  reactions 
of  travelers  upon  the  environments  in  which  they  journey 
or  linger.  Many  epidemics  of  infectious  diseases  have 
sprung  from  germs  left  by  travelers,  and  most  of  the 
great  plagues  and  pestilences  of  history  have  followed  the 
routes  of  pilgrims,  caravans,  crusaders,  conquerors,  traders, 
or  travelers.  "  Walking  cases  "  of  typhoid  fever,  diphthe- 
ria, etc.,  are  perhaps  most  dangerous  to  the  public  health, 
and  tramps,  peddlers,  and  other  roving  characters  do  much 
to  spread  disease.  Persons  coming  down  with  an  infec- 
tious disease,  such  as  typhoid  fever,  are  very  apt  to  leave 
off  work  and  go  a-fishing,  sometimes  upon  or  along  the 
shores  of  a  public  water  supply,  which  they  may  unwit- 
tingly contaminate.  Life  away  from  home  has  its  dangers 
for  the  traveler;  it  is  no  less  true  that  life  at  home  has 
its  dangers,  these  often  arising  from  travelers  themselves. 

10.  Public  Conveyances,  because  they  are  used  promis- 
cuously by  the  well  and  the  ailing  alike,  are  subject  to 
infection,  and  for  this  reason  carriages,  cars,  and  steamboats 
should  be  kept  clean  and  occasionally  should  be  thoroughly 
disinfected.    Steamboats    and   steamships    are    essentially 
floating  hotels,  and  should  be  treated  as  such.    Sleeping 
cars  bear  less  resemblance  to  public  houses,  and  may  be 
cleaned  partly  by  washing,  partly  by  blasts  of  compressed 
air,  and  partly  by  disinfectants,  and  when  properly  cared  for 
are  less  likely  to  endanger  health  than  are  many  hotels. 
Their  lavatories  should  be  kept  scrupulously  clean  and 
should  be  frequently  disinfected.    In  modern  times  vast 
improvements  have  been  made  in  all  kinds  of  public  con- 
veyances in  the  direction  of  greater  steadiness,  less  noise, 
better  heating,  better  air,  and  better  lighting.    The  public 
drinking  cup,  however,  remains  as  objectionable  as  ever, 
and  its  use  ought  to  be  abolished. 

11.  Public  Houses.  —  Hotels  and  other  public  houses 
may  be  either  clean,  wholesome,  and  restful,  or  unclean, 


HYGIENE  OF  TKAVELING  527 

noisy,  and  unsanitary.  Owing  to  the  fact  that  their  popu- 
lation is  constantly  changing  they  are  far  more  exposed  to 
infection  than  are  private  houses,  and  great  pains  should 
be  taken  to  keep  them  always  in  good  sanitary  condition. 
The  simplest  (iron)  bedsteads  are  the  best,  and  in  hotels  all 
carpets,  draperies,  etc.,  should  either  be  avoided  or  sub- 
jected to  frequent  and  thorough  cleaning.  The  kitchen, 
especially,  requires  careful  supervision  to  insure  cleanliness, 
and  in  the  laundry  the  linen  should  be  so  treated  as  to  be 
sterilized  during  the  process  of  washing.  Employees  should 
be  instructed  in  the  art  of  cleanliness,  and  any  suffering 
from  contagious  or  infectious  diseases  should  be  excluded 
or  quarantined.  All  lavatories  should  be  kept  scrupulously 
clean  and  should  be  frequently  disinfected. 

12.  Public  Places,  such  as  streets,  parks,  playgrounds, 
and  cemeteries,  are  dangerous  only  when  infected.    Dirty 
streets  are  unsightly  and  disagreeable,  but  it  is  very  hard  to 
trace  the  source  of  much  disease  directly  to  them.    Never- 
theless, few  things  sooner  or  more  agreeably  impress   a 
visitor  than  clean  streets,  and  in  the  lower  portions  of  the 
town  or  city  clean  streets  are  particularly  important  because 
the  streets  are  the  home  and  the  playground  of  the  children 
of  the  poor.    Pavements  in  cities  should  be  hard  and  non- 
absorbent  rather  than  porous,  and  should  be  kept  clean 
and  free  from  rubbish. 

13.  Public  Parks  are  desirable  for  fresh  air,  recreation, 
rest,  and  change  of  scene,  and  in  these  respects  are  impor- 
tant hygienic  factors  in    city  life.    They  are   of   special 
benefit  to  those  living  in  tenement  houses  or  under  crowded 
conditions.    Public  playgrounds  minister  to  the  needs  of 
the  same  class  of  people.    Their  importance  can  hardly  be 
overestimated,  since  they  fucnish  to  city  children  almost 
the  sole  opportunity  for  normal  physical  development  and 
some  contact  with  nature.    It  has  been  well  said  that  "  the 
boy  without  a  playground  is  the  father  of  the  man  without 


528  THE  HUMAN  MECHANISM 

a  job."    But  here  also  wise  supervision  and  cleanliness  are 
the  conditions  of  hygienic  success. 

Public  cemeteries  in  America  are  usually  well  conducted 
and  unobjectionable  from  the  hygienic  standpoint.  The 
objections  sometimes  urged  against  them  as  centers  of 
infection  and  sources  of  disease  are  seldom  well  founded. 
Cremation,  or  the  burning  of  the  dead,  is  slowly  but  stead- 
ily growing  in  favor  and  has  much  to  recommend  it  from 
the  sanitary  standpoint,  since  it  prevents  slow  decay  and 
destroys  completely  all  germs  of  disease.  Near  most  of 
the  larger  American  cities  there  are  now  one  or  more 
crematories. 


CHAPTER  XXXIV 
PUBLIC  PROTECTION  OF  THE  PUBLIC  HEALTH 

1.  The  Public  Health.  —  By  this  term  is  meant  the 
health  of  the  community;  and  of  some  community  every 
family  and  every  individual  is  a  member.  The  public 
health  is  obviously  of  vital  importance  to  the  individual, 
and,  conversely,  the  health  of  the  individual  is  of  vital 
importance  to  the  community.  Personal  hygiene,  or  the 
hygiene  of  the  individual,  and  public  hygiene,  or  the 
hygiene  of  the  community,  are  thus  closely  bound  together. 
Not  only  because  it  is  his  duty,  but  also  because  from  the 
selfish  point  of  view  it  is  to  his  advantage,  the  individual 
should,  therefore,  interest  himself  in  and  seek  to  promote 
the  public  health.  If,  for  example,  smallpox  appears  in 
his  community,  he  cannot  afford,  even  from  a  selfish  point 
of  view,  to  fail  to  do  his  best  to  aid  in  suppressing  it. 
Conversely,  if  he  himself  falls  ill  of  smallpox,  his  neigh- 
bors and  the  whole  public  naturally  feel  a  similar  interest 
in  isolating  him  and  preventing  the  spread  of  the  disease. 

For  these  and  similar  reasons,  people  living  in  commu- 
nities, and  especially  in  villages,  towns,  and  cities,  by  com- 
mon consent  usually  elect  or  appoint  a  few  of  their  own 
number  as  sanitary  authorities  or  officials  to  attend  to 
matters  affecting  the  public  health.  The  citizens  thus 
chosen  are  endowed  with  special  powers  and  privileges, 
and,  taken  together,  are  generally  designated  as  the  Board 
of  Health,  or  Health  Commissioners.  Sometimes,  especially 
in  small  communities,  there  is  no  formally  organized  board 
of  health,  the  duties  of  such  a  board  being  performed  by 

529 


530  THE  HUMAN  MECHANISM 

some  governing  body  of  the  community,  such  as  the 
selectmen,  etc. 

2.  Boards  of  Health,  their  Powers  and  Duties.  —  Very 
much  as  boards  of  police  are  chosen  by  the  people  to  pre- 
serve public  order  and  to  prevent  disturbance  and  crime, 
so  boards  of  health  are  chosen  to  preserve  the  public 
health  and  prevent  disease  and  death.  And  as  the  police 
officer  could  not  possibly  do  the  work  assigned  to  him 
without  unusual  powers  and  privileges,  these  sometimes 
involving  a  considerable  interference  with  personal  liberty, 
so  the  health  officer  cannot  do  the  work  expected  of  him 
without  unusual  powers  and  privileges.  But  it  should 
never  be  forgotten  that  in  each  case  both  the  officers 
themselves  and  the  powers  which  they  possess  exist  by 
the  common  consent  of  the  community,  which  desires,  and 
thus  provides  for,  protection  at  the  cost  of  surrendering 
some  personal  rights  and  privileges.  Boards  of  this  sort 
derive  their  powers  solely  from  the  consent  of  the  majority 
of  the  community  which  they  serve,  and  those  members  of 
the  community  who  disapprove  of  their  existence,  powers, 
and  acts  must  either  persuade  the  majority  to  do  other- 
wise, or  must  submit,  or  must  go  elsewhere. 

Among  the  important  powers  of  boards  of  health  are  the 
right  of  quarantine,  isolation,  entrance  and  search,  and 
vaccination.  A  board  of  health  may,  in  many  cities  of 
the  United  States,  detain  a  vessel,  perhaps  full  of  pas- 
sengers impatient  of  delay  and  eager  to  land,  at  quarantine, 
even  for  many  days,  subjecting  the  owners,  passengers,  and 
others  to  great  inconvenience,  expense,  and  damage.  A 
board  of  health,  finding  smallpox  in  a  hotel  or  boarding 
house,  may  quarantine  or  isolate  the  building,  surround  it 
by  police,  and  forbid  all  persons  to  enter  or  leave  it,  thus 
causing  great  alarm  and  annoyance  to  the  inmates,  great 
damage  to  the  proprietor,  and  a  heavy  expense  to  the  com- 
munity. A  board  of  health  may  declare  general  vaccination 


PUBLIC  PROTECTION  OF  PUBLIC  HEALTH    531 

necessary  for  the  protection  of  the  public  health,  and  may 
even  enforce  vaccination  upon  the  careless,  reluctant,  or 
resisting.  It  may  forbid  a  dairyman  to  sell  milk  thought 
to  contain  typhoid  fever  or  other  disease  germs,  thus  caus- 
ing the  dairyman  great  inconvenience  and  even  financial 
ruin.  In  all  these  cases  the  board  is,  as  a  rule,  simply 
obeying  the  wishes  of  a  majority  of  the  community,  and 
those  who  are  delayed,  constrained,  or  financially  injured 
have  to  submit  as  best  they  may,  unless  the  general  senti- 
ment of  the  community  undergoes  a  change  in  their  favor. 
The  duties  of  boards  of  health  are  manifold.  Some  of  the 
most  obvious  and  general  are  usually  prescribed  by  public 
statute  or  ordinance.  Such,  for  example,  are  the  following 
in  the  state  of  Massachusetts : 

The  State  Board  of  Health  shall  take  cognizance  of  the  interests 
of  health  and  life  among  the  citizens  of  the  Commonwealth.  It  shall 
make  sanitary  investigations  and  inquiries  in  respect  to  the  causes 
of  disease,  and  especially  of  epidemics  and  the  sources  of  mortality, 
and  the  effects  of  localities,  employments,  conditions,  and  circum- 
stances on  the  public  health ;  and  shall  gather  such  information  in 
respect  to  those  matters  as  it  may  deem  proper,  for  diffusion  among 
the  people.  It  shall  advise  the  government  in  regard  to  the  location 
and  other  sanitary  conditions  of  any  public  institutions. 

Others  are  less  general  and  more  specific,  like  the  fol- 
lowing : 

The  State  Board  of  Health  shall  have  the  general  supervision  of 
all  streams  and  ponds  used  by  a  city  or  town  as  sources  of  water 
supply,  with  reference  to  their  purity,  together  with  all  springs, 
streams,  and  water  courses  tributary  thereto  ;  and  shall  have  authority 
to  examine  the  same  from  time  to  time  a.nd  inquire  what  pollutions 
exist  and  what  are  their  causes. 

When  the  Board  of  Health  of  any  city  or  town  has  had  notice  of 
the  occurrence  of  a  case  of  smallpox  or  of  any  other  disease  dan- 
gerous to  the  public  health  in  such  city  or  town,  such  Board  of  Health 
shall,  within  twenty-four  hours  after  the  receipt  of  such  notice,  notify 
the  State  Board  of  Health  of  the  same. 


THE  ROLLS"  MECHANISM 

A  nation  may  and  should  have  a  national  sanitary 
authority  charged  with  the  protection  and  promotion  of 
the  public  health  and  provided  with  large  power- 
should  also  be  supplied  with  trained  experts  and  money 
enough  to  enable  these  to  deal  with  emergencies,  to  study 
large  practical  problems,  and  to  carry  on  researches  into  the 
causes  of  disease  and  the  improvement  of  methods  for  their 
prevention.  Germany  has  such  an  organization  in  its  Impe- 
rial Board  of  Health,  and  the  United  States,  for  a  short 
time,  had  a  National  Board  of  Health.  At  present  the 
United  States  Public  Health  and  Marine  Hospital  Service 
does  this  same  work  to  some  extent. 

The  states  also,  in  the  United  States,  have  for  the  most 
part  their  own  boards  of  health;  but  such  boards  do  not, 
as  a  rule,  have  very  large  powers,  these  being  reserved  for 
the  so-called  u  local  boards  "  of  the  various  cities  and  towns, 
believed  by  many  experts  that  a  larger  grant  of  powers 
and  resources  to  state  and  national  boards  would  be  of 
immense  benefit  to  the  public,  and  would  secure  for  all  a 
much  more  constant  and  efficient  sanitary  protection. 

3.  What  the  Individual  may  do  to  protect  the  Public 
Health.  —  The  first  duty  of  the  individual  to  the  public- 
health  is  to  remember  that  he  himself,  his  family,  his  house, 
and  all  his  belongings  constitute  one  important  and  funda- 
mental element  in  the  health  of  the  community  of  which 
he  is  a  unit.  He  should  therefore  seek,  first  of  all, 
to  maintain  and  promote  good  health  in  himself,  in  his 
family,  and  in  all  his  household;  for  the  prevention  of 
disease  and  premature  death  in  one  household  is  a  distinct 
and  genuine  contribution  to  the  public  health  of  all  other 
households. 

In  the  next  place,  he  should  cheerfully  conform  to  all 
reasonable  regulations  of  the  board  of  health  or  other  sani- 
tary authority  of  his  community,  duly  prescribed  by  them 
under  powers  conferred  by  the  community  as  a  whole. 


PUBLIC  PKOTECTION  OF  PUBLIC  HEALTH     533 

Finally,  he  should  inform  himself  as  fully  and  as  accu- 
rately as  possible  upon  hygienic  and  sanitary  subjects,  in 
order  not  only  to  protect  and  promote  his  own  health  and 
that  of  his  household,  but  also  to  enable  him  to  become 
an  intelligent,  critical,  and  yet  cooperative  member  of  the 
community,  thus  doubly  aiding  in  preserving  and  promot- 
ing the  public  health. 

Having  done,  or  tried  to  do,  these  three  things,  the  good 
citizen  has  still  one  further  duty  of  the  utmost  importance 
to  perform  for  the  maintenance  and  betterment  of  the 
public  health,  —  which,  as  we  have  shown  above  is  also 
of  great  consequence  both  to  himself  and  his  family,  —  and 
that  is  to  aid  and  assist  in  all  their  good  works  boards  of 
health  and  all  others  in  sanitary  authority.  This,  one  may 
do  by  reporting  the  existence  of  cases  of  infectious  disease, 
nuisances,  etc. ;  by  trying  to  secure  the  election  or  appoint- 
ment of  intelligent,  upright,  and  expert  officials  ;  by  loyally 
upholding  such  officials  in  the  performance  of  their  duty ; 
by  refusing  to  countenance  opposition  to  necessary  public 
procedures  such  as  vaccination,  gas  inspection,  plumbing 
inspection,  the  placarding  of  houses  containing  cases  of 
infectious  disease,  the  isolation  of  patients,  etc.,  and  in 
many  other  ways  which  are  certain  to  arise. 

At  times  this  individual  responsibility  for  public  health 
involves  personal  inconvenience  and  hardship,  severely 
testing  the  good  citizenship  even  of  those  most  desirous 
of  cooperating  with  the  public  health  authorities.  This  is 
well  illustrated  in  the  case  of  diphtheria.  When  antitoxin 
is  given  in  this  disease,  the  toxin  produced  by  the  bacteria 
is  neutralized  in  the  blood  and  tissues  and  is  thus  pre- 
vented from  injuring  the  organism ;  but  not  all  the  bacteria 
are  immediately  killed  by  this  treatment.  Consequently  it 
sometimes  happens  that,  long  after  the  clinical  symptoms 
have  disappeared  and  when  the  patient  is  apparently  per- 
fectly normal,  examination  of  the  throat  reveals  the  presence 


534  THE  HUMAN  MECHANISM 

of  the  bacillus ;  and  it  has  been  proved  beyond  question 
that  germs  from  this  source  are  often  capable  of  transmit- 
ting the  disease  to  healthy  persons.  It  is  a  real  hardship 
to  such  a  patient  to  be  kept  in  quarantine  for  days  and 
weeks,  until  the  disappearance  of  the  germ  in  the  throat 
is  established  by  bacteriological  examination,  and  boards 
of  health  are  frequently  criticised  severely  for  enforcing 
quarantine  under  such  circumstances;  but  it  is  obvious 
that  these  measures  are  demanded  in  the  interests  of  the 
community  and  that  resistance  to  them  can  arise  only  from 
ignorance  or  selfishness,  or  both. 

4.  What  the  Public  may  do  to  protect  and  promote  the 
Health  of  the  Individual.  —  On  the  other  hand,  the  com- 
munity, through  its  paid  or  unpaid  officials,  may  do  much 
to  protect  and  promote  the  health  of  its  individual  mem- 
bers. It  may  see  to  it  that  the  public  water  supply,  if  any, 
is  pure;  it  may  maintain  an  efficient  system  of  milk  in- 
spection; it  may  provide  investigations  of  food  adultera- 
tion, and  prosecutions  and  penalties  for  the  same  ;  it  may 
require  prompt  and  efficient  scavenging,  and  the  collec- 
tion and  removal  of  wastes  such  as  sewage,  garbage,  and 
other  refuse  ;  it  may  establish  a  wholesome  system  of  school 
hygiene  ;  it  may  prevent  the  concealment  of  the  existence 
of  cases  of  infectious  or  contagious  disease  ;  it  may  pro- 
vide for  vaccination  against  smallpox,  and  for  the  use  of 
antitoxic  serum  in  diphtheria ;  and  in  many  other  ways  it 
may  protect  the  individual  and  his  family  even  better  than 
he,  unaided,  could  protect  himself. 


CHAPTER  XXXV 
THE  HEALTH  OF  NATIONS 

1.  The  Modern  World  One  Vast  Community.  —  Ever  since 
the  invention  of  the  mariner's  compass,  followed  as  this 
was  by  the  voyages  of  discovery  of  Columbus,  Vasco  da 
Gama,  and  Magellan,  the  world  has  become,  century  by 
century,  more  and  more  one  great  community  or  neigh- 
borhood.    With  the  introduction  of  steam  transportation 
on  land  and  sea  our  globe  has  practically  shrunk  so  that 
intercourse  between  the  various  nations  of  the  earth  has 
become  both  frequent  and  easy,  and  if  no  other  means  than 
those  formerly  known  existed  for  the  prevention  of  disease, 
plagues  and  pestilences  would,  without  question,  ravage 
mankind  worse  than   ever   before.    The   isolation  of  the 
ancient  world  was  its  sanitary  salvation,  but  to-day  there 
is  no  isolation.     Steamers   ply   regularly  and   frequently 
between  Orient  and  Occident,  commingling  the  people  and 
the  products  of  the  whole  world.    Books,  newspapers,  let- 
ters, food  materials,  fabrics,  and  many  other  sorts  of  mer- 
chandise pass  freely  back  and  forth,  and  yet  plague  and 
pestilence  to-day  seldom  follow  in  their  train.    It  is  possi- 
ble, and  even  probable,  that  some  milder  diseases  such  as 
influenza,  or  the  grippe,  may  still  owe  their  sudden  and 
wide  distribution  to  the  modern  ease  and  extent  of  com- 
munication;  but  mails  and  steamers   come  and  go,   and 
bubonic  plague,  and  Asiatic  cholera,  and  smallpox,  though 
occasionally  brought  by  them  to  Europe  or  America,  do 
not  make  great  headway  there  after  their  arrival. 

2.  Ancient  Paths  of  Pestilence  and  Plague.  —  Although 
it  is  true  that  modern  civilization  is  indebted  to  the  Orient 

535 


536  THE  HUMAN  MECHANISM 

for  its  first  knowledge  of  the  art  of  inoculation  for  the 
prevention  of  smallpox,  it  is  no  less  true  that  many  of  its 
worst  epidemic  diseases  have  often  come  from  the  same 
source. 

The  plague,  a  world-famous  disease  (p.  502),  has  afflicted 
mankind  for  centuries,  and  has  repeatedly  appeared  in 
Europe,  traveling  westward  from  the  Orient  and  from 
Africa.  The  Black  Death,  which  is  held  to  have  destroyed 
one  fourth  of  the  population  of  Europe  in  the  fourteenth 
century,  was  probably  a  virulent  form  of  the  oriental 
plague  which  entered  Europe  from  the  south  and  east. 
The  Great  Plague  of  London  (in  1665)  probably  came 
from  Holland,  in  bales  of  merchandise  brought  from  the 
Levant. 

The  Asiatic  cholera,  as  its  name  suggests,  has  repeat- 
edly come  to  Europe  and  America  from  the  East,  and  is 
believed  to  exist  almost  constantly  in  India,  from  which 
place  its  germs  are  readily  conveyed  to  western  countries. 
The  germs  of  the  great  Hamburg  (Germany)  epidemic  of 
1892  were  probably  brought  there  by  immigrants  from 
Russia. 

3.  The  Modern  Impotence  of  Pestilence  and  Plague.  - 
The  modern  increase  of  the  means  of  communication  has 
no  doubt  tended  to  spread  far  and  wide  all  sorts  of  conta- 
gious and  infectious  diseases,  but  with  that  increase  there 
has  come,  especially  within  the  last  few  years,  such  a 
scientific  knowledge  of  these  diseases  and  of  the  ways  of 
holding  them  in  check  that,  in  spite  of  vastly  greater  facil- 
ities for  their  distribution,  they  are  actually  less  dangerous 
to  mankind,  and  far  less  dreaded,  than  formerly.  The 
appearance  of  the  bubonic  plague  in  China  or  in  India,  or  of 
Asiatic  cholera  in  Japan  or  in  the  Philippines,  still  causes 
international  anxiety,  and  vigorous  local  precautionary  or 
corrective  measures  are  taken  to  overcome  them  ;  yet  little 
widespread  alarm  is  felt.  The  closer  intimacy  between 


THE  HEALTH  OF  NATIONS  537 

Cuba  and  the  United  States  since  the  Spanish  War  of 
1898,  while  in  itself  favoring  the  spread  of  yellow  fever, 
has  had  the  marvelous  and  happy  consequence  —  thanks 
to  the  brilliant  researches  and  able  administration  of  the 
medical  and  sanitary  officers  of  the  American  army  —  not 
of  bringing  more  yellow  fever  to  the  United  States,  as 
would  formerly  have  been  the  case,  but  of  virtually  extir- 
pating that  disease,  for  the  present  at  least,  in  Cuba. 

4.  The    Use    and   Abuse    of    Quarantine.  —  The    word 
"  quarantine "    comes    from    the   French    word    quarante, 
meaning  forty,  because  a  detention  of  forty  days  was  for- 
merly enforced  upon  travelers  crossing  frontiers.    Quar- 
antine is  of  great  value  in  some  cases,  as,  for  example,  in 
ports  like   Boston   or  New  York,   and  when  thoroughly 
enforced  may  be  an  important  means  of  protecting  the 
country  against  infectious  disease.    When  a  vessel  which 
has  been  long  enough  at  sea  to  give  contagious  disease  (if 
present)  time   to  appear,   comes  into  port  with  cases   of 
such  disease  on  board,  its  detention  is  a  wise  precaution. 
On  the    other   hand,   indiscriminate  quarantine   between 
states  or  cities,  or  of  vessels  that  have  come  from  near 
ports,  so  that  little  or  no  time  has  been  given  for  disease, 
if  present,  to  show  itself,  is  necessarily  severe  and  often 
useless  and  unwarrantable. 

Quarantine  is  also  liable  to  abuse  on  other  grounds,  for 
it  is  claimed  that  it  has  sometimes  been  unjustifiably 
employed  to  keep  out  of  a  country  foods  or  other  products 
which  came  into  competition  with  domestic  products,  the 
plea  of  sanitary  danger  being  raised  for  commercial  reasons. 

5.  International  Sanitary  Congresses.  —  From  time  to 
time  there  are  held  nowadays  international  sanitary  con- 
gresses which  undertake  to  deal  with  the  larger  questions 
affecting  the  health  of  nations.    There  is  also  held  annually 
an  International  Congress  of  Hygiene  and  Demography; 
and  the  annual  meetings  of  the  American  Public  Health 


538  THE  HUMAN  MECHANISM 

Association,  in  which  the  United  States,  Canada,  Mexico, 
and  Cuba  are  represented,  are  really  international  con- 
gresses for  a  large  part  of  the  western  hemisphere. 

6.  Health  and  Longevity  in  Various  Countries.  —  It  is 
interesting  to  inquire  how  different  nations  compare,  one 
with  another,  in  respect  to  health  and  longevity.  It  might 
be  supposed  that  somewhere  on  the  earth's  surface  the 
climate  should  be  so  salubrious,  the  food  so  wholesome, 
the  conditions  so  favorable,  and  life  so  normal,  that  sick- 
ness would  be  unknown  and  death  indefinitely  postponed. 
Invalids  in  large  numbers  do,  in  fact,  turn  to  Colorado  or 
California,  to  Madeira  or  to  the  Riviera,  seeking  in  these 
places  more  favorable  conditions  for  sustaining  or  prolong- 
ing life  ;  but  no  place. has  ever  been  found  altogether  free 
from  disease,  and  no  climate,  however  salubrious,  seems 
capable  of  causing  any  great  increase  in  longevity.  It 
was  many  centuries  ago  in  the  Orient,  and  of  a  race  singu- 
larly strong  and  persistent,  that  the  Jewish  poet  wrote 
those  majestic  lines  which  for  every  land  and  every  people 
are  no  less  true  to-day :  "  The  days  of  our  years  are  three- 
score years  and  ten  ;  arid  if  by  reason  of  strength  they  be 
fourscore  years,  yet  is  their  strength  labor  and  sorrow; 
for  it  is  soon  cut  off  and  we  fly  away." 

The  general  death  rate  —  that  is,  the  number  of  deaths 
per  year  per  thousand  of  the  population  —  is  not  a  com- 
plete measure  either  of  health  or  of  longevity,  but  is  some- 
times the  only  test  we  have,  and  the  following  table  for 
1900  shows  how  great  is  the  difference  in  the  death  rates 
of  some  of  the  larger  cities  of  the  world. 

London 18.7      Moscow 30.0 

New  York 20.6      Rome 16.5 

Paris 20.5      Madrid 33.3 

Berlin 18.9      Stockholm 17.1 

Vienna 20.6      Boston 20.8 

St.  Petersburg      ....  27.0 


THE  HEALTH  OF  NATIONS 


539 


The  following  table  (from  the  United  States  Census  of 
1900)  gives  the  death  rates  for  the  periods  specified  of 
some  of  the  principal  countries  of  the  civilized  world. 

COMPARATIVE  DEATH  RATES  PER  1000  POPULATION  FOR 
CERTAIN  COUNTRIES 


1890 

TWENTY-FIVE 
YEARS 
1876-1900 

1900 

Austria/  .     . 

29  4 

28  6 

25  4 

Belgium           .           .... 

20  6 

20.1 

19  3 

19.0 

18.3 

16.9 

England  and  Wales      

19.5 

19.1 

18.2 

France  . 

22*8 

21.9 

21  9 

German  Empire       .... 

24  4 

24  2 

22  1 

Prussia  

24.0 

23.7 

21.8 

Hungary 

32  4 

32  3 

26  9 

Ireland 

18  2 

18  2 

19  6 

Italy      

26.4 

26  5 

23  8 

20.5 

20.3 

17.8 

Norway 

17  9 

16  6 

15  9 

Scotland    ... 

19  7 

19  2 

18  5 

32.5 

30.3  * 

28.7 

Sweden      
Switzerland 

17.1 

20  8 

17.0 
20  6 

16.8 
19.3 

19  6 

17  8 

7.  The  Sanitation  of  the  World.  —  Enough  has  been  said 
in  the  foregoing  paragraphs  to  show  that  while  the  hopes 
of  dreamers  seeking  after  an  elixir  of  life  have  no  founda- 
tion, and  while  a  wholly  salubrious  environment  cannot 
greatly  prolong  human  life  beyond  the  usual  period,  mucji 
is  being  done  and  much  still  remains  to  be  done  for  a 
more  complete  and  perfect  sanitation.  Infectious  diseases 


Average  for  twenty  years,  1878-1884,  1888-1900. 


540  THE  HUMAN  MECHANISM 

still  sweep  over  communities,  carrying  sickness  and  death 
among  the  people,  increasing  the  death  rate,  and  diminish- 
ing longevity.  Here  and  there  nations  and  individuals  are 
devoting  themselves  with  energy,  public  spirit,  and  wisdom 
to  investigation  of  the  causes  of  disease,  and  to  improve- 
ment of  the  environment  by  careful  organization  of  boards 
of  health,  by  municipal  sanitation,  by  sanitary  engineer- 
ing, by  purer  water  and  milk  supplies,  by  proper  sewerage 
and  sewage  disposal,  by  food  inspection,  and  the  like.  All 
this  is  wise  and  encouraging,  but  it  is  only  a  beginning. 
Far  more  might  and  ought  to  be  done  both  by  nations 
and  by  individuals.  Many  of  the  nations,  especially  those 
known  as  half-civilized  or  barbarous,  have  as  yet  hardly 
made  a  beginning  in  hygiene  or  sanitation,  and  as  long 
as  this  is  the  case  they  are,  and  will  continue  to  be,  a  men- 
ace not  only  to  themselves  but  to  the  whole  world,  which, 
as  one  vast  community,  is  in  these  respects  closely  bound 
together. 

The  student  should  never  forget  that  the  foundation  of 
municipal,  national,  and  international  hygiene  and  sani- 
tation, and  therefore  of  the  health  of  nations,  ultimately 
rests  upon  the  hygiene  and  sanitation  of  individuals, — that 
is,  upon  personal  hygiene  and  sanitation.  If  all  human 
beings  were  healthy  and  clean,  the  nations  of  the  world 
would  of  necessity  be  in  the  same  condition.  Personal 
hygiene  and  personal  sanitation  thus  form  the  basis  of 
all  hygiene  and  sanitation,  whether  of  home  or  village, 
of  town  or  city,  or  of  the  world ;  and  the  essentials  of 
personal  hygiene  and  sanitation  are  simply  the  proper 
management  and  care  of  the  human  mechanism  and  its 
surroundings. 


FIG.  134.  The  thoracic  and  abdominal  cavities,  after  the  removal  of 
the  organs  shown  in  Fig.  2 

The  diaphragm  has  been  drawn  somewhat  forward 


I  — 


FIG.  135.  General  view  of  the  digestive  tract.    After  Spalteholz 

A,  mouth  cavity ;  B,  pharynx;  C,  oesophagus ;  D,  diaphragm;  E,  stomach ; 
F,  small  intestine ;  G,  ascending  colon ;  H,  descending  colon ;  /,  rectum. 
The  transverse  colon  has  been  cut  away,  its  position  being  indicated  by 
dotted  lines 


FIG.  136.  The  flouncelike  folding  of  the  mesentery,  as  seen  after 
'  removing  the  small  intestine.    After  Spalteholz 


A-l- 


FIG.  137.  Median  dorso-ventral  section  of  the  trunk  in  the  abdominal 
region,  showing  the  suspension  of  the  stomach  and  intestine  by  the 
mesentery.  After  Spalteholz 

A,  liver;    B,  stomach;    C,  transverse  colon;   D,  mesentery;  E,  rectum; 
F,  urinary  bladder 


FIG.  138.  The  permanent  teeth  in  the  jaw-bones,  viewed 
from  the  right.    After  Spalteholz 


FIG.  139.  The  network  of  capillaries  on  the  lining  of  the 
air  cells  of  the  lungs.    After  Kolliker 

See  page  167 


A (J 


—  E 


FIG.  140.  First  layer  of  muscles  of  the  breast  and  shoulder  region. 
After  Spalteholz 

A,  biceps  of  the  arm  (p.  34) ;  B,  deltoid ;  C,  portion  of  the  trapezius  (see 
Figs.  106  and  107) ;  D,  clavicle ;  E,  sternum  or  breastbone ;  F,  pecto- 
ralis  major  (see  p.  324  and  Fig.  107) 


I) 


FIG.  141.  Second  layer  of  muscles  of  the  breast,  exposed  by  dissecting 
away  the  pectoral  is  major  in  Fig.  140.    After  Spalteholz 

A,  B,  the  two  "  heads  "  of  the  biceps  ;  C,  cut  end  of  the  pectoralis  major ; 
I),  deltoid;  E,  pectoralis  minor ;  F,  trapezius ;  G,  clavicle;  H,  first  rib; 
K,  sternum.  Note  the  direct  attachment  of  the  intercostal  muscles  to 
the  ribs  (p.  8).  Compare  Fig.  140 


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The  parts  of  the  nervous  system 
represented  are  the  cerebrum, 
cerebellum, bulb, and  segment 
of  the  spinal  cord.    Afferent 
nerves  in  red,  efferent  nerves 
in  black,    m,  m,  motor  neu- 
rones to  some  of  the  muscles 
of  the   leg.     These   may  be 
stimulated  to  coordinate  ac- 
tion   by    neurones  (v)   from 
the  cerebrum,  neurones  (cb) 
from  the  cerebellum,  or  by 
the   afferent   neurones  (a/1) 
from  the  tendons,  etc.    In  the 
bulb   this   afferent    neurone 
connects  with  a  second  neu- 
rone (a/2),  and  this  with  a 
third  (a/3),    thus  providing 
the  path  to  the  cerebrum  and  exciting 
in  consciousness  sensations  of  position 
of  the  leg  (muscular  sense).    The  same 
neurones  connect  with  the  cerebellum, 
as  do  also  neurones  from  the  inner  ear. 
For  further  explanations  see  Chapter 
XV,  pp.  279-282. 


FIG.  145.  Diagram  of  the  nervous  mechanism  of  walking 


\ 


FIG.  146.  Side  view  of  the  brains  of  rabbit,  cat,  and  monkey 
See  page  271 


<u 

2  i 

2  * 

I  s 

1  fa> 

'S  fe 

S  .3 


^  'So 

2  g 

'I  i 

I  a 


INDEX 


Abdominal  breathing,  170. 

Abdominal  cavity.  See  Peritoneal 
cavity. 

Abdominal  muscles,  action  in 
breathing,  172. 

Absorption  from  the  intestine,  123. 

Accommodation,  for  near  objects, 
248 ;  muscle  of,  247. 

Adipose  tissue,  182,  230. 

Adrenal,  68. 

Adulteration  of  foods,  508. 

Afferent  impulses,  77  ff. ;  reflex 
and  conscious  effects  of,  281. 

Afferent  neurones,  77,  78. 

Air  cell,  168. 

Air,  good  and  bad,  pure  and  im- 
pure, 444 ;  change  of,  522.  See 
also  Fresh  air. 

Albuminoids,  91,  218. 

Alcohol,  physiological  action  of, 
366 ;  as  a  stimulant,  367,  369  ;  as 
a  food,  372 ;  as  a  defense  against 
cold,  372  ;  pathological  conditions 
due  to,  373 ;  in  muscular  work, 
370 ;  influence  on  self-control, 
375. 

Alcoholic  beverages,  composition  of, 
363. 

Alimentary  canal,  structure  of,  20 ; 
as  a  digestive  laboratory,  97. 

Alimentation,  88,  96. 

Alveolus  of  gland,  31,  32  (fig.),  33 
(fig.);  of  lungs,  167,  168  (fig.). 

Amoeba,  amoeboid  movement,  135. 

Amylopsin,  119. 

Anesthetics,  284. 

Animal  foods,  94,  111,  240,  241. 


Ankle,  bones  of,  19,  403  (fig.),  404 

(fig-)- 

Anopheles,  490,  491  (fig.),  492  (fig.). 

Anterior,  9. 

Antiseptics,  475,  504 ;  in  food  pres- 
ervation, 512. 

Antitoxin,  488,  499. 

Aorta,  13,  22,  24  (fig.),  25,  141. 

Apical  lobes  of  lungs.    See  Lungs. 

Appendicular  skeleton,  14,  19. 

Appetite,  265 ;  as  a  guide  in  feed- 
ing, 348. 

Aqueduct  of  Sylvius,  268,  270. 

Aqueous  humor,  247. 

Arborization.    See  Synapse. 

Arterial  reservoir,  139. 

Arterial  tone,  158. 

Arteries,  22,  38,  144  (fig.). 

Asphyxia,  168. 

Astigmatism,  253. 

Auditory  nerve,  28,  259. 

Augmentor  nerves  of  heart,  157. 

Auricle,  22,  24  (fig.),  137. 

Auriculo-ventricular  valves,  138. 

Automatic  nervous  actions,  82. 

Axial  skeleton,  14. 

Axon  or  axis  cylinder,  73,  77. 

Bacilli,  469. 

Bacteria,  469.    See  also  Microbes. 

Balance  exercises,  331. 

Balance  movements,  276. 

Bathing  and  baths,  413  ff.,  476. 

Beds  and  bedsteads,  431. 

Bile,  119. 

Bile  duct,  107  (fig.). 

Bladder,  urinary,  179,  180  (fig.). 


553 


554 


THE  HUMAN  MECHANISM 


Blood,  arterial  and  venous,  23, 163 ; 
microscopic  structure  of,  133; 
gases  oi,  163 ;  as  a  common  car- 
rier, 132 ;  distribution  among 
organs,  144. 

Blood  corpuscles,  red,  133-135 ;  as 
carriers  of  oxygen,  164. 

Blood  corpuscles,  white,  133,  134 ; 
behavior  during  inflammation, 
382. 

Blood  plasma,  134,  135 ;  gases  of, 
163. 

Blood  vessels,  38.  See  also  Arteries, 
Capillaries,  and  Veins. 

Boards  of  Health.    See  Health. 

Body  cavity,  10. 

Bone,  37.    See  also  Skeleton. 

Brain,  27 ;  of  frog,  267 ;  of  mam- 
mal, 270 ;  histological  structure 
of,  271 ;  the  seat  of  sensations, 
245 ;  functions  of,  274  ff. 

Breastbone,  17. 

Breathing  movements,  168,  169; 
effect  on  circulation,  145,  173; 
effect  on  flow  of  lymph,  148, 173  ; 
hygiene  of,  172 ;  in  muscular  ac- 
tivity, 308,  315. 

Breathlessness,  174. 

Bronchiole,  168. 

Bronchitis,  381. 

Bronchus,  12,  21,  167. 

Bulb,  267  (fig.),  268,  270  (fig.); 
functions  of,  275. 

Caffeine,  361. 

Calendered  paper,  399. 

Calorie,  212. 

Canal,  spinal  or  vertebral,  18. 

Canning  of  foods,  512. 

Capillaries,  25  (fig.),  27,  32  (fig.), 
39. 

Capsule  of  gland,  31. 

Carbohydrates,  91,  218  ;  digestion 
of,  103,  110,  119;  waste  products 
of,  177 ;  as  source  of  power  for 
work,  213 ;  fuel  value  of,  213 ;  as 


food  in  cold  climates,  216 ;  as 
source  of  fat,  231. 

Carbon  dioxide  (carbonic  acid) 
formed  during  muscular  work, 
49,  60,  175,  306;  in  lymph,  162; 
in  blood  plasma,  163. 

Cardiac  region  of  stomach,  107. 

Carpets,  432. 

Catalytic  actions,  106. 

Catarrh,  chronic,  394. 

Catarrhal  conditions,  care  of,  383. 

Cellar,  sanitation  of,  428. 

Cell  walls  in  plants,  94. 

Cells,  32,  33,  36,  38,  40,  41  (fig.), 
43 ;  as  chemical  factories,  52  ; 
as  living  mechanisms,  63 ;  waste 
and  repair  of,  220. 

Cellulose,  94. 

Cement  of  tooth,  99. 

Cemeteries,  528. 

Central  canal  of  spinal  cord,  268. 

Cerebellum, 267 (fig.), 268,  273  (fig.). 

Cerebrum,  83,  266  (fig.),  267  (fig.), 
269  (fig.),  270  (fig.);  connections 
with  other  parts  of  the  nervous 
system,  279;  functions  of,  277, 
283. 

Cervical  vertebrae,  14. 

Cesspools,  459. 

Chemical  change,  relation  to  work, 
52 ;  as  source  of  heat,  193 ;  as 
influenced  by  temperature,  187. 

Chicken  pox,  493,  494. 

Chloral,  377. 

Chloroform,  284. 

Chocolate,  362. 

Cholera,  Asiatic,  501. 

Choroid,  247,  248,  251  (fig.). 

Chyme,  112. 

Cilia,  474. 

Ciliary  muscle,  248. 

Ciliary  region  of  eye,  248  (fig.). 

Cinders,  removal  from  eyes,  400. 

Circulation,  organs  of,  22,  132, 
140  (fig.);  time  of ,  136;  mechan- 
ics of,  136;  in  warm  and  cold 


INDEX 


555 


weather,  150,  200;  during  mus- 
cular activity,  151;  during  diges- 
tion, 155,  156 ;  during  mental 
work  and  sleep,  155  ;  nervous  fac- 
tors of,  156  ff . ;  essential  to  respi- 
ration, 174 ;  in  foot,  409. 

Cisterns,  454. 

Clavicle,  20. 

Cleanliness,  sanitary  value  of,  433. 

Climate,  and  mental  work,  204  ;  and 
feeding,  236 ;  in  treatment  of  tu- 
berculosis, 482. 

Clothing,  hygiene  of,  418  ff. 

Coagulation  of  proteids,  91. 

Coarse  foods,  importance  of,  355. 

Cocaine,  377. 

Cocci,  469,  470  (fig.). 

Coccygeal  vertebrae,  14. 

Cochlea,  259. 

Cocoa,  362. 

Coffee,  361. 

Cold,  effect  on  circulation  of  the 
blood,  150 ;  effect  on  body  as  a 
whole,  200  ff . 

Cold,  sensations  of,  261. 

Cold-blooded  animals,  189. 

Colds,  nature,  prevention,  and  care 
of,  380  ff . 

Collagen,  91,  218. 

Collar  bone.    See  Clavicle. 

Collaterals,  79. 

Colon,  21. 

Color,  sensations  of,  256. 

Compensatory  adjustments  of  the 
circulation,  150. 

Conduction  of  heat,  194-196. 

Congestion  of  blood  in  internal 
organs,  307  ;  during  inflamma- 
tion, 382. 

Connective  tissues,  structure  of,  7, 
8,  37,  228,  230 ;  of  glands,  31,  32  ; 
of  muscles,  34,  35  ;  of  nerves,  72  ; 
of  lungs,  167  ;  of  skin,  182 ;  re- 
lation to  blood  vessels,  38  ;  to 
lymphatics,  39  ;  digestion  of,  109, 
115  (footnote). 


Consciousness,  244,  275,  277,  283, 

285. 
Consciousness  of  correct  posture, 

education  of,  326. 
Constant  temperature  of  the  body, 

188. 

Constipation,  130. 
Consumption.    See  Tuberculosis. 
Contagious  diseases.    See  Diseases. 
Contraction  of  muscle,  34,  48,  56. 
Convection  of  heat,  194,  195. 
Convolutions  of  cerebrum,  270  (fig), 

271. 

Cooking,  243 ;  as  an  aid  in  nutri- 
tion,  349;    sterilization  of  food 

by,  511. 

Cooling  off  suddenly,  391. 
Coordination,  70,  80,  275  ;  training 

of,  86,  280,  330. 
Corium,  182. 

Cornea,  247,  248  (fig.),  251  (fig.). 
Corpuscles.    See  Blood  corpuscles. 
Corrective  muscular  exercises,  321 ; 

hygienic  value  of,  332. 
Cortex  of  cerebrum  and  cerebellum, 

272. 

Costal  breathing,  170. 
Cotton  underwear,  421. 
Cough  medicines,  387. 
Course  dinners,  242. 
Cranial  nerves,  271. 
Cranium,  18. 
Creatinin,  178. 
Culex,  492  (fig.). 
Curd  of  milk,  90,  108. 
Curvatures  of  vertebral  column,  16. 
Curve  of  fatigue,  58. 
Cutaneous  sensations,  261. 
Cutis,  182. 
Cycling,  317. 
Cytoplasm,  32,  36,  43,  76. 

Dampness,  380. 

"  Dangerous    region  '•    of    atmos- 
pheric temperature,  201,  390. 
Deafness,  401. 


556 


THE  HUMAN  MECHANISM 


Dendrites,  76,  77,  272,  273,  274. 

Dentine,  99. 

Dermis,  182. 

Dextrines,  92,  105. 

Dextrose,  98. 

Diaphragm,  10 ;  action  in  respira- 
tion, 170-172. 

Diarrhea,  130,  380,  455. 

Diastase,  364. 

Diastole,  137,  139  (fig.). 

Diet,  226. 

Digestibility  of  food,  226. 

Digestion,  organs  of,  20  ;  nature  of, 
88,  98  ;  external  and  internal,  97  ; 
in  the  mouth,  98  ;  in  the  stomach, 
105  ;  in  the  intestine,  115 ;  cooper- 
ation of  processes  of,  129  ;  and 
the  circulation,  155  ;  and  temper- 
ature regulation,  205 ;  as  affected 
by  muscular  activity,  309 ;  dur- 
ing fatigue,  342. 

Diphtheria,  486,  533. 

Diseases,  293  ;  mental  cures  of,  345; 
infectious  and  contagious,  467  ; 
prevention  of  microbic,  475. 

Disinfectants,  475. 

Distilled  liquors,  365.  ' 

Divisive  movements  of  intestine, 
121. 

Dorsal,  9. 

Drainage  in  house  sanitation,  428, 
458. 

Drinking  cups,  public,  525. 

Driving  force  for  circulation  through 
capillaries,  144. 

Drug  habit,  358. 

Drugs,  357  ff.  ;  in  the  operation  of 
the  nervous  system,  344  ;  in  the 
catarrhal  conditions,  387 ;  as  a 
substitute  for  hygienic  living, 
388;  in  treatment  of  tubercu- 
losis, 482. 

Ducts  of  glands,  20,  29,  32  (fig.), 

33  (fig.). 
Dust  as  vehicle  of  tuberculosis,  479. 

Dyspepsia,  113. 


Ear,  structure  of,  258  ;  care  of,  401. 
Efferent  nerve  fibers  and  impulses, 

76,  80  (fig.). 

Elasticity  of  arteries,  142. 
Elimination    of    intestinal    waste, 

128. 

Emmetropia,  250. 
Enamel,  99. 
End  organs  of  nerves,  28,  76-84, 

245. 

Engine,  analogy  of,  62. 
Enjoyment  of  food,  hygienic  value 

of,  114. 
Enjoyment  of  muscular  activity, 

311. 

Environment,  299. 
Enzyme,   45,  48,  52;    action  of, 

101  ff. ;  of  saliva,  103  ;  of  gastric 

juice,    108  ;   of   small   intestine, 

118,  120. 

Epidemics,  466.    See  also  Plagues. 
Epidermis,  182. 
Epiglottis,  20  (fig.). 
Equilibrium,    nervous    factors    in, 

276  ;  in  physical  training,  330. 
Esophagus.    See  (Esophagus. 
Ether,  284. 

Eustachian  tube,  259,  401. 
Excitation,  285,  367. 
Excretion,  177  ;  in  relation  to  feed- 
ing, 225  ;  essential  and  incidental 

organs  of,  178. 
Extractive,  135,    218. 
Eye,  structure  of,  246 ;  care  of,  395. 

Face,  bones  of,  18. 

Farsightedness,  253. 

Fasciculus  of  muscle,  34,  35. 

Fatigue,  55  ff.  ;  hygienic  value  of, 
66 ;  influence  on  digestion,  353  ; 
and  taking  cold,  389  ;  practical 
considerations  concerning,  310. 

Fatigue  level,  58,  61. 

Fats,  92,  218  ;  of  meat,  115  ;  diges- 
tion of,  119  ;  waste  products  of, 
177 ;  fuel  value  of,  213  ;  as  food 


INDEX 


557 


in  cold  climates,  217  ;  storage  of, 
224,  227,  229,  231  ;  a  hindrance 
to  output  of  heat,  205. 

Fatty  acids,  92,  119. 

Feces,  128. 

Feeding,  muscular  activity  after, 
156 ;  effect  on  chemical  changes 
in  the  body,  221 ;  for  muscular 
activitjr,  heat,  cold,  mental  work, 
and  sedentary  occupations,  235- 
237;  hygiene  of,  347;  during 
colds,  386. 

Femur,  19. 

Fermentation,  364. 

Fever,  210. 

Fibula,  19. 

Filters,  house,  455. 

Filth  and  filth  diseases,  484. 

Filtration  of  public  water  supplies, 
513  f. 

Fires,  open,  196,  434. 

Flagella,  474  (fig.). 

Flat  foot,  406. 

Flesh  of  body,  227;  increase  of, 
228. 

Food  accessories,  357  ff. 

Food  stuff.    See  Nutrient. 

Foods,  chemical  composition  of,  89, 
95 ;  animal  and  vegetable,  94, 
240 ;  as  source  of  energy  or  power, 
88,  211 ;  as  material  for  growth 
and  repair,  88, 220-222;  fuel  value 
of,  211 ;  heating,  216;  choice  of, 
233;  adulteration  and  infection 
of,  508  ff.;  preservatives  and 
antiseptics  in,  511;  canning  of, 
512. 

Foot,  hygiene  of,  403  ff.  ;  arches 
of,  404;  deformation  of,  405; 
physical  training  of,  410. 

Force-pump  action  of  heart,  137, 
139  (fig.). 

Forebrain,  267  (fig.),  283. 

Fresh  air,  as  a  substitute  for  exer- 
cise, 320 ;  as  a  cure  for  colds,  385. 

Fried  foods,  352. 


Fuel  substances,  storage  of,  in  mus- 
cle, 49. 
Fuel  value  of  food,  211. 

Gall  bladder,  107  (fig.). 

Games,  318. 

Ganglion,  73 ;  of  the  dorsal  root, 

77. 

Garbage,  disposal  of,  461. 
Gas,    illuminating,    440 ;    natural, 

coal,  and  water,  515 ;  poisoning 

by,  516. 
Gaseous   exchange    in   capillaries, 

164,  165 ;  during  muscular  activ- 
ity, 175. 

Gastric  digestion,  350. 
Gastric  juice,  108 ;  secretion  of,  113. 
Gelatin,  91. 

General  muscular  exercise,  314, 332. 
Germ  theory  of  disease,  467. 
Germicides,  475. 
Glands,    20,    29,  31,  32   (fig.),    33 

(fig.);  working  and  resting,  44, 

47 ;  blood  supply  during  activity, 

45 ;  ductless,  29,  68. 
Glottis,  20  (fig.). 
Gluten,  90. 
Glycerin,  92. 
Glycogen,  227,  232. 
Granules,  storage  of,  in  gland  cells, 

47. 
Gray   matter  of  spinal  cord   and 

brain,  75. 
Grippe,  493. 
Ground  water,  451,  513. 
Growth,  favorable  period  for  the 

acquisition  of  deformities,  325. 
Gums  or  dextrines,  92. 
Gymnasium,  332,  476. 

Habits,  physical  basis  of,  287. 
Hair   and    hair   follicle,    183,    185 

(fig.)- 

"  Hardening"  to  cold,  392. 
Harvey,  William,  136. 
Headaches,  128,  396. 


558 


THE  HUMAN  MECHANISM 


Health,  293 ;  public,  529 ;  national, 
state,  and  local  boards  of,  530 ;  in 
different  countries,  538.  See  also 
Public  health. 

Hearing,  258. 

Heart,  10,  12,  22;  valves  of,  138; 
nerves  of,  157  ;  effect  of  muscular 
activity  on,  307. 

Heart  beat,  136 ;  force-pump  action 
of,  137,  139  (fig.);  regulation  of, 
157. 

Heat,  effect  of,  on  the  circulation 
of  the  blood,  150;  unit  of,  212; 
transfer  of,  from  internal  organs 
to  the  skin,  195 ;  production  and 
transfer  of,  192,  198;  supply  of 
energy  for  production  of,  216 ; 
as  a  food  preservative,  512. 

Heating  foods,  216. 

Hemoglobin,  134,  135,  165. 

Hepatic  artery,  24  (fig.). 

Hepatic  vein,  24  (fig.),  27. 

Hind  brain,  267. 

Hip  bones,  20. 

Hoarseness,  21. 

Horny  layer  of  skin,  182,  183. 

Hotels,  sanitation  of,  526. 

House,  sanitation,  425  ff. ;  construc- 
tion, 429;  furnishings,  430; 
floors,  431 ;  care  of,  432. 

Humerus,  19,  20. 

Humidity,  influence  on  temperature 
regulation,  202;  influence  on 
mental  work,  204. 

Hunger,  264. 

Hygiene,  scope  and  subdivisions 
of,  301 ;  personal,  304  ;  domestic, 
425 ;  public,  463. 

Hypermetropia,  253,  255. 

Hypnotics,  344. 

Ice  supply  of  the  house,  455. 
Illumination  for  near  work,  255. 
Illusions,  optical,  257. 
Immunity,   natural  and  artificial, 
497. 


Indigestible  material  in  food,  93. 

Indigestion,  113,  125. 

Infection  of  foods,  509. 

Infectious  diseases.    See  Diseases. 

Inflammation,  381. 

Influenza,  493. 

Inhibition,  158,  367 ;  in  the  nerv- 
ous system,  285,  367 ;  an  active 
process  in  muscular  relaxation, 
343. 

Inhibitory  nerves  of  heart,  157. 

Inorganic  salts.    See  Salts. 

Instep,  bones  of,  19. 

Interdependence  of  organs,  65. 

Internal  secretion,  67. 

Intestinal  juice,  117,  120. 

Intestinal  waste,  elimination  of, 
128. 

Intestine,  small,  10,  21,  24  (fig.), 
26,  116;  large,  11,  21,  127;  action 
of  muscular  coat  of,  120. 

Iris,  247,  248  (fig.),  251  (fig.). 

Irritability,  46,  50. 

Jugular  vein,  24  (fig.). 
Junket  tablets,  108. 

Kidneys,  11,  13,  24  (fig.),  26,  68; 

structure  of,  179,  181. 
Kilogrammeter,  212. 
Koch,  Robert,  473,  478. 

Labyrinth  of  ear,  260. 

Large  intestine.    See  Intestine. 

Laryngitis,  381. 

Larynx,  21. 

Lateral  costal  breathing,  171. 

Lateral  curvature  of  spine,  329. 

Lens  of  eye,  247,  248  (fig.),  251 

(fig.) ;    formation   of   image  by, 

249,  250  (fig.). 
Levulose,  98. 
Ligaments,  8,  14,  16,  17. 
Lighting  of  the  house,  439. 
Linen  underwear,  421. 
Lipase,  119. 


INDEX 


559 


Liver,  11,  21,  24  (fig.),  26,  27,  29, 
68,  107  (fig.),  117. 

Lobes  and  lobules  of  glands,  30,  31 
(fig.);  of  the  lung,  13,  172. 

Lockjaw,  501. 

Locomotion,  nervous  factors  in, 
276,  277,  280. 

Longevity,  538. 

Lumbar  vertebrae,  14. 

Lungs,  10,  12,  21,  24  (fig.);  struc- 
ture of,  167 ;  apical  lobes  of, 
172,  309. 

Lymph,  39 ;  origin  of,  40 ;  environ- 
ment of  cells,  40,  135  ;  gases  of, 
162. 

Lymph  flow,  function  of,  42 ;  cause 
of,  137 ;  influenced  by  respira- 
tory movements,  173 ;  influenced 
by  muscular  activity,  308. 

Lymph  spaces,  39. 

Lymphatics,  41,  147. 

Malaise,  128,  396. 

Malaria,  489  ff. 

Malt  liquors,  364. 

Massage,  147. 

Master  neurones,  84. 

Mastication,  hygienic  aspects  of, 
349. 

Mattings,  432. 

Measles,  493,  494. 

Meat,  a  proteid  food,  90 ;  composi- 
tion, 220;  in  diet,  242;  eating 
of,  350  ;  as  a  vehicle  of  infection, 
479. 

Mediastinum,  11,  21. 

Medulla  oblongata.     See  Bulb. 

Mental  cures  of  disease,  345. 

Mental  states,  influence  on  the 
health  of  the  nervous  system, 
344. 

Mental  work  and  the  circulation, 
155;  after  meals,  156,  353;  as 
influenced  by  climatic  condi- 
tions, 204;  feeding  for,  237; 
effect  on  nutrition,  237. 


Mesentery,  12,  26. 

Meshwork  underwear,  421. 

Microbes,  467,  468 ;  as  scavengers, 
471 ;  in  decomposition  and  decay, 
472 ;  as  disease  germs,  473 ; 
growth,  multiplication,  and  spore 
formation,  473  (fig.). 

Microbic  action  on  food,  225. 

Microbic  life  in  the  intestine,  125, 
128. 

Micromillimeter,  or  micron,  133. 

Microparasites,  467. 

Microscopic  work  and  the  eyes, 
399. 

Midbrain,  267  (fig.). 

Milk,  108 ;  as  a  vehicle  of  infection, 
479,  517,  531. 

Milk  supplies,  purity  of,  517.     • 

Moisture,  influence  on  temperature 
regulation,  202. 

Monosaccharides,  92. 

Moral  conduct  as  a  part  of  nervous 
hygiene,  344. 

Morphine,  376. 

Mosquito,  as  transmitter  of  malaria, 
491 ;  as  transmitter  of  yellow 
fever,  493. 

Motor  nerves,  28. 

Mouth,  183. 

Movements,  active  or  passive,  effect 
on  circulation,  146  ;  respiratory. 
See  Breathing  movements. 

Mucin,  45,  127. 

Mucous  coat  of  stomach,  108  ;  of 
intestine,  116,  127. 

Muscle  fibers,  35,  36  ;  of  stomach 
and  intestine,  108,  116,  127  ;  of 
arteries  and  veins,  144. 

Muscles,  8,  16  ;  structure  of,  34  ; 
isolated,  49,  56;  physiology  of, 
48,  56  ;  and  temperature  regula- 
tion, 207  ;  antagonistic  action  of, 
17,  208  ;  in  faulty  carriage,  322  ff. 

Muscular  activity,  effect  on  circu- 
lation, 146,  151,  156;  on  respira- 
tion, 175  ;  and  the  regulation  of 


560 


THE  HUMAN  MECHANISM 


the  temperature  of  the  body,  203, 
390,  423 ;  hygiene  of,  304 ;  minis- 
try to  body  as  a  whole,  304  ;  after 
meals,  156, 353 ;  and  fatigue,  316 ; 
for  women,  313 ;  physiology  of, 
306. 

Muscular  exercises,  general  charac- 
ter of  the  most  useful,  312  ;  for 
special  purposes,  321. 

Muscular  sense,  relations  to  loco- 
motion and  maintenance  of  equi- 
librium, 280. 

Muscular  work,  power  for,  213  ; 
feeding  for,  235 ;  of  stomach, 
110  ;  of  intestine,  120. 

Myopia,  252. 

Narcotics.  344.  See  also  Anes- 
thetics. 

Nasal  cavity,  20. 

Nearsightedness,  252. 

Near  vision,  249,  254,  396. 

Neck,  carriage  of,  328. 

Nerve  cells,  73  ff.,  271. 

Nerve  fibers,  structure  of,  73 ;  affer- 
ent and  efferent,  76,  77 ;  of  spinal 
cord  and  brain,  75,  271. 

Nerve  roots,  75. 

Nerves,  28 ;  distribution  to  organs, 
42  ;  structure  of,  72  ;  cranial,  271. 

Nervous  impulse,  nature  of,  87;  in 
reflexes  and  sensation,  246,  266. 

Nervous  strain,  335  ff . 

Nervous  system,  general  anatomy 
of,  26,  27,  266  ;  structural  organi- 
zation of,  72  ff. ;  training  by  prac- 
tice, 86,  286  ;  physiology  of,  70, 
273 ;  and  the  circulation,  157- 
161  ;  and  respiration,  173  ;  and 
secretion  of  perspiration,  185; 
and  regulation  of  body  tempera- 
ture, 209  ;  and  carriage  of  body, 
324  ;  hygiene  of,  334. 

Nervousness,  396. 

Neurones,  77;  "master,"  84;  of 
brain,  272. 


Nitrogenous  equilibrium,  223. 

Noise,  401. 

Non-proteids,  218 

Nucleus,  32,  36. 

Nutrients,  89 ;  classification  of,  218 ; 
special  effects  of  each  on  oxida- 
tions of  the  body,  221  ;  on  flesh 
weight,  222  ;  on  storage  of  fat, 
224  ;  choice  of,  233,  239. 

Nutrition,  211  ff. 

(Esophagus,  10,  12,  20  (fig.),  183. 

Opium,  376. 

Optic  lobes,  267. 

Optic  nerve,  28,  247,  251  (fig.). 

Organs,   typical    structure    of,   41 

(fig.),  42. 
Overfeeding,  351. 
Overheating  of  houses,  439. 
Oxidation,    49,    50,    52,    92,    162; 

products  of,  177. 
Oxygen,   role  of,   in  cell  life,  48, 

162 ;   in   lymph,    162  ;  in   blood 

plasma,    163 ;    absorbed    during 

muscular  activity,  175. 

Pain,  sensations  of,  263  ff.  ;   signs 

of,  284. 
Palate,  20. 
Pancreas,  anatomical  relations,  11, 

21,  26,  29,  107   (fig.),   117  ;  the 

source  of  an  internal  secretion, 

68. 

Pancreatic  duct,  107. 
Pancreatic  juice,  109,  118. 
Paper,  in  printing,  399. 
Papillae  of  skin,  182. 
Parasites,  467. 
Parks,  476,  527. 
Parotid  gland,  30. 
Pasteur,  Louis,  473. 
Pasteurization,  475. 
Pelvis,    of  skeleton,    19   (fig.),    20 

(footnote)  ;   of  ureter,  180,   181 

(fig-)- 
Pepsin,  108,  118. 


INDEX 


561 


Peptones,  98. 

Peristalsis,  122. 

Peritoneal  cavity,  10,  11,  12. 

Peritoneum,  10,  11,  13. 

Peritonitis,  11. 

Perspective,  idea  of,  256. 

Perspiration,  composition  of,  179 ; 
secretion  of,  184  ;  sensible  and 
insensible,  185  ;  and  the  output 
of  heat,  197  ;  in  relation  to  feed- 
ing, 352. 

Pharyngitis,  381. 

Pharynx,  20,  21,  183,  259. 

Plague,  bubonic,  502. 

Plagues,  466,  535. 

Plasma.    See  Blood  plasma. 

Play,  287,  318. 

Playgrounds,  476. 

Pleura,  10,  11. 

Pleural  cavity,  10, 11 ;  pressure  in, 
169. 

Pleurisy,  11. 

Plumbing  of  the  house,  456. 

Pons  Varolii,  270  (fig.). 

Portal  vein,  24  (fig.),  27,  125. 

Position,  sense  of,  262,  280,  327. 

Posterior,  9. 

Posture,  nervous  factors  in,  276 ; 
faulty,  as  a  cause  of  deformity, 
322. 

Presbyopia,  253. 

Preservatives  of  food,  511. 

Pressure  in  arteries  and  veins,  142, 
143,  152  (fig.);  in  pleural  space, 
169. 

Processes  of  nerve  cells,  76. 

Proteids,  nature  of,  90, 115,  218 ;  in 
blood  plasma,  135  ;  in  living  cells, 
220  ;  digestion  of,  in  stomach  and 
intestine,  109, 118  ;  fuel  value  of, 
213 ;  quantity  in  body,  222 ;  waste 
products  of,  177;  influence  on 
secretion  of  urine,  181 ;  disinte- 
gration of,  221;  native,  126. 

Proteoses,  109. 

Protozoa,  469,  470. 


Pseudopodium,  134. 

Psychic  secretion  of  gastric  juice, 

113,  350. 
Ptyalin,  103. 
Public   health,   463  ff.;  rules  and 

regulations,  464  ;  authorities,  465; 

problems,  465. 
Pulmonary  arteries,  22,  24  (fig.), 

25, 167. 

Pulmonary  circulation,  23, 140  (fig.) . 
Pulmonary  veins,  22,  23,  24  (fig.), 

25, 167. 

Pulp  cavity,  of  tooth,  99. 
Pupil  of  eye,  247. 
Purposeful  character  of  reflex  and 

volitional  actions,  71,  80,  275. 
Pylorus,  107  (fig.). 

Quarantine,  530,  537;  in  tubercu- 
losis, 480 ;  in  smallpox,  496 ;  in 
diphtheria,  533. 

Radiation  of  heat,  195,  196. 

Radius,  19. 

Railroad  trains,  reading  on,  399. 

Rectum,  130,  183. 

Reflexes,  81 ;  of  locomotion,  etc., 

280 ;    disappearance   of,    during 

anesthesia,  284. 

Relaxation,  muscular,  in  sleep,  342. 
Renal  arteries,  26. 
Rennin,  108,  118. 
Repair  of  cells,  220. 
Reservoirs,    arterial    and    venous, 

139. 
Resistance  to  the  flow  of  blood, 

143,  152  (fig.). 
Respiration,  organs  of,  21 ;  of  the 

cells,162;  nervous  factors  in,  173; 

and  muscular  activity,  175,  308, 

315. 
Respiratory  movements.  SeeBreath- 

ing  movements. 
Rest,  in  relaxation  and  sleep,  334, 

338 ;  in  change  of  work,  340. 
Retina,  247. 


562 


THE  HUMAN  MECHANISM 


Rheumatism,  380. 

Rhinitis,  381. 

Ribs,  17  ;  action  in  respiration,  170, 

172. 
Rugs,  432. 

Sacrum,  14,  20. 

Saliva,  chemical  composition  of,  45 ; 
secretion  of,  44-48 ;  action  in  di- 
gestion, 101,  105. 

Salivary  glands,  21,  30. 

Salts,  inorganic,  45,  93,  218,  219. 

Sanitation,  scope  and  subdivisions 
of,  301;  domestic,  425;  public, 
303,  463. 

Saprophyte,  467 1 

Sarcolactic  acid,  49,  60. 

Sarcolemma,  35. 

Sarcostyles,  36,  53. 

Scapula,  20. 

Scarlet  fever,  494. 

Sclerotic  coat,  247,  248. 

Sebaceous  glands,  184. 

Secondary  aids  to  the  circulation, 
145. 

Secretion,  general  physiology  of, 
44-48;  internal,  67;  of  gastric 
juice,  113;  of  pancreatic  juice, 
bile,  and  intestinal  juice,  116-120; 
of  urine,  180;  of  perspiration, 
184. 

Sedentary  occupations,  237. 

Segmenting  movements  of  intestine, 
121. 

Semicircular  canals,  259,  260  (fig.), 
263,  280. 

Sensations,  244,  245,  246;  refer- 
ence of,  244. 

Sense  organs,  78,  244,  245. 

Sense  of  position,  importance  of,  in 
physical  training,  327. 

Septum,  of  gland,  31 ;  of  muscle, 
34. 

"Setting-up"  drill,  326. 

Sewage,  disposal  of,  459,  519. 

Shivering,  208. 


Shoes,  405,  407 ;  for  deformed  feet, 
409 ;  temperature  and  moisture 
within,  410. 

Shoulder  blade,  20. 

Shoulder  girdle,  20. 

Sinuses  of  temporal  bone,  260. 

Skeleton,  14,  15,  19. 

Skin,  7 ;  structure  and  functions, 
181 ;  regulator  of  the  output  of 
heat,  199 ;  as  an  organ  of  absorp- 
tion, 186 ;  care  of,  185,  413. 

Skull,  19. 

Sleep,  circulation  during,  153;  hy- 
giene of,  334  ff. 

Smallpox,  495 ;  inoculation  and  vac- 
cination for,  498. 

Smell,  sensations  of,  261. 

Soaps,  92,  119. 

Soda  water,  363. 

Soil,  an  element  in  house  sanita- 
tion, 428. 

Solidity,  ideas  of,  256. 

Somnambulism,  278. 

Speech,  282. 

Spinal  column,  14,  17 ;  faults  of 
carriage  of,  328. 

Spinal  cord,  gross  anatomy  of,  18, 
27;  structure  of,  74,  267;  func- 
tions of,  274. 

Spirilla,  469,  471  (fig.). 

Spitting  nuisance,  488. 

Spleen,  11,  26. 

Sporozoa,  470. 

Springs,  453. 

Sputum,  as  vehicle  of  tuberculosis, 
479. 

Starch,  91,  94  (fig.);  digestion  of, 
102,  110,  119.  See  also  Carbo- 
hydrates. 

Steapsin.    See  Lipase. 

Stegomyia,  493. 

Sterilization,  475 ;  of  food  by  cook- 
ing, 511. 

Sternum.    See  Breastbone. 

Stimulants,  344,  357  ff. 

Stimulation,  46,  52,  80,  81. 


INDEX 


563 


Stomach,  anatomical  relations  of, 
10,  21,  26;  structure  of,  107  ff .  •; 
digestive  work  of,  108-115. 

Storage  of  material  in  the  cell,  52. 

Stoves,  oil  and  gas,  437. 

Streets,  sanitation  of,  527. 

Submaxillary  gland,  30. 

Submucous  coat  of  intestine,  116 
(fig.),  127  (fig.). 

Suction  action  of  breathing  move- 
ments, 148. 

Sugars,  92,  104,  105,  119.  See  also 
Carbohydrates. 

Summer  complaint  in  children,  495. 

Sunshine,  428. 

Supplies,  public,  of  food,  water,  and 
gas,  505  ff. 

Supporting  organs  and  tissues,  37, 
220. 

Suprarenal,  68. 

Surface  water,  451,  513. 

Suspensory  ligament  of  eye,  248. 

Sweat  glands,  structure  of,  184. 

Synapse,  79,  273  (fig.). 

Systemic  circulation,  23. 

Systole,  137. 

Tannic  acid,  361. 

Tartar,  100. 

Taste  sensations,  261. 

Tea,  361. 

Teeth,  98. 

Temperature,  external,  influence 
on  chemical  change  and  vital  ac- 
tivities, 187,  188 ;  reactions  of 
body  to  changes  of,  200;  "dan- 
gerous region"  of,  201,  390;  of 
living  rooms,  390,  439,  447 ;  in- 
fluence on  secretion  of  urine, 
181. 

Temperature  of  the  body,  188-205. 

Temperature  sensations,  192,  195, 
262. 

Tendon,  8,  34,  35. 

Tetanus,  501. 

Theine,  361. 


Thermal  phenomena  of  the  body, 

187. 

Thirst,  264. 

Thoracic  cavity.  See  Pleural  cavity. 
Thyroid  glands,  66. 
Tibia,  19. 
Tobacco,  377. 
Toes,  bones  of,  19;  flexion  of,  in 

hygiene  of  foot,  405. 
Tone,    arterial,    158;    of    skeletal 

muscle,  209. 
Touch,  261 . 
Trachea,  10,  11,  21. 
Traveling,  hygiene  and  sanitation 

of,  520  ff. 
Trichina,  510. 
Trunk  movements,  315. 
Trypsin,  118. 
Tuberculosis,  477  ff. 
'Tweenbrain,  267  (fig.). 
Tympanic  membrane,  258, 259  (fig. ) . 
Tympanum,  259. 
Type,  size  of,  398. 
Typhoid  fever,  483  ff. 

Ulna,  19. 

Underclothing,  420. 
Urea,  177. 

Ureter,  179,  181  (fig.). 
Uric  acid,  178. 
Urine,  secretion  of,  180. 
Use  and  disuse  in  the  training  of 
the  nervous  system,  286. 

Vaccination,  498,  530. 

Valves,  of  heart,  22,  138,  141;  in. 

veins,  146. 

Vasoconstrictor  nerves,  159. 
Vasodilator  nerves,  160. 
Vasomotor.      See    Vasoconstrictor 

and  Vasodilator. 

Vegetable  foods,  94,  111,  240,  350. 
Vegetarianism,  240. 
Veins,  7,  23,  26,  38,  39,  144  (fig.) ; 

intermittent  compression  of,   in 

muscular  activity,  146. 


564 


THE  HUMAN  MECHANISM 


Vense  cavae,  13,  22,  24  (fig.),  26. 

Venous  reservoir,  139. 

Ventilation,  442  ff.;  natural,  443; 
mechanical  systems  of,  449. 

Ventral,  9. 

Ventricles,  of  heart,  22,  24, 137  ;  of 
brain,  268,  269. 

Vertebra,  14,  16. 

Vertebral  column.  See  Spinal  col- 
umn. 

Villus  of  intestine,  116  (fig.),  117, 
118  (fig.),  123  (fig.). 

Visual  judgments,  256. 

Visual  sensations,  255. 

Vital  resistance,  297,  482. 

Vitreous  humor,  247,  251  (fig.). 

Volitional  actions  or  movements, 
82,  279,  282. 

Walking  as  a  means  of  exercise, 

319.    See  Locomotion. 
Wandering  cells,  134. 
Warm  weather,  and  feeding,  156 ; 

circulation  in,  150. 
Warm-blooded  animals,  189. 
Warming   of   the  house,   434  ff. ; 

by  open  fire,  434 ;  by  stoves,  435 ; 

by    hot-air    furnaces,    435 ;    by 

steam  and  hot  water,  437. 
Warmth,  sensations  of,  261. 
Waste  products,  49,  52,  59,  65, 

177  ;  excretion  of,  225. 


Water,   as   a  food,  93,  218,   219; 

and  the  secretion  of  urine,  181 ; 

use  as  a  drink,  354. 
Water  supply,  of  the  house,  451; 

purity  of  public,  513,  526. 
Waters,  hard  and  soft,  454. 
Wells,  452. 
White  matter  of  spinal  cord  and 

brain,  75. 

Whooping  cough,  493. 
Will,  82. 
Wind  and  temperature  regulation, 

203. 

Wines,  365. 
Winking,    muscular    and    nervous 

mechanism  of,  70. 
Woolen  underwear,  421. 
Work,  unit  of,  212. 
Wort,  365. 

Wounds,  care  of,  503. 
Wrist,  bones  of,  19. 
Writing,  283. 

Xanthin  bases,  178. 

Yeast,  364. 
Yellow  fever,  492. 

Zymogen,  48. 


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